Purchase Order vs Invoice: Differences, Workflow and Best Practices

Purchase orders and invoices are two of the most important documents in business-to-business trade. They look similar, both list items, quantities and prices, but they come from opposite sides of the transaction, at different times, for different reasons. Understanding how they work together is the foundation of a controlled purchasing process and of getting paid on time as a supplier.

This guide explains what a purchase order and an invoice are, how they differ, how they fit into the procure-to-pay cycle, how matching works, how each is treated in accounting, and the mistakes that cause disputes and late payments. Contract law and tax rules vary between jurisdictions, so treat legal points here as general.

Definitions

Purchase order (PO): a document the buyer sends to a supplier to order goods or services. It specifies what is being bought, quantities, prices, delivery dates and terms. Internally, raising a PO usually requires approval, which makes it a spending control. When the supplier accepts it, expressly or by delivering, the PO generally becomes part of the contract between them.

Invoice: a document the seller sends to the buyer after supplying goods or services, requesting payment. It references what was supplied, the amounts due, taxes and payment terms. See what is an invoice for full details.

Side-by-side comparison

Aspect	Purchase order	Invoice
Issued by	Buyer	Seller
Sent to	Seller	Buyer
Timing	Before the supply	After the supply (or a milestone)
Purpose	Order and authorize a purchase	Request payment
Effect	Commits the buyer once accepted	Creates a payable for the buyer, a receivable for the seller
Key reference	PO number	Invoice number, quoting the PO number
Accounting entry	Usually none (a commitment, not a liability)	Recorded as payable/receivable (accrual accounting)
Tax role	Usually none	Often the tax invoice for VAT/GST
Can change	By PO amendment before fulfilment	Corrected by credit note, not edited

Where each fits in the procure-to-pay process

A typical procure-to-pay (P2P) cycle:

1. Need identified: an employee needs goods or services.
2. Requisition: an internal request, approved according to policy.
3. Purchase order: procurement issues a PO to the chosen supplier.
4. Supplier acknowledgement: the supplier confirms the order, price and delivery.
5. Delivery and receipt: goods arrive and are checked; a goods received note (GRN) or receiving record is created. For services, someone confirms completion.
6. Invoice: the supplier invoices, quoting the PO number.
7. Matching: AP compares invoice, PO and receipt.
8. Approval: discrepancies resolved; invoice approved for payment.
9. Payment: made according to terms.
10. Reconciliation: payment matched on the bank statement, and the supplier account reconciled.

The PO comes at step 3; the invoice at step 6. Between them, delivery happens, and the record of that delivery is what lets the buyer check the invoice properly.

What a purchase order contains

• The words "Purchase Order" and a unique PO number.
• Buyer's company details and delivery address.
• Supplier's details.
• PO date and required delivery date.
• Item descriptions, codes, quantities and unit prices.
• Total value, with tax shown or noted.
• Payment terms and delivery terms (such as Incoterms for international goods).
• Approver details, in some systems.
• Terms and conditions, or a reference to them.
• Instructions for invoicing, such as "Quote PO number on all invoices; send to [email protected]".

Types of purchase orders

Type	Use
Standard PO	One-off purchase with known items, quantities and prices
Planned PO	Known items and prices, delivery dates scheduled later
Blanket PO	Agreed spend limit or prices with a supplier over a period, with releases against it
Contract PO	Framework terms with a supplier; specific orders reference it

Blanket POs suit recurring purchases like office supplies or maintenance, where raising a PO for every order would be inefficient.

Matching: how POs and invoices work together

Before paying, AP teams compare documents. The level of matching depends on the purchase:

• Two-way matching: invoice vs PO. Checks the supplier billed for what was ordered at the agreed price.
• Three-way matching: invoice vs PO vs goods receipt. Adds the check that what was billed was actually received.
• Four-way matching: adds inspection or quality acceptance.

Our guide to three-way matching explains tolerances, exceptions and automation in detail.

Worked example

A buyer issues PO-5521 for 200 units of a component at 4.50, total 900.00, plus 20% tax. The warehouse receives 180 units. The supplier invoices 200 units at 4.60, total 920.00 plus tax.

• Price check: 4.60 vs 4.50 — exceeds the agreed price.
• Quantity check: billed 200 vs received 180 — 20 units not received.

The correct amount is 180 × 4.50 = 810.00 plus tax. AP holds the invoice and asks the supplier for a credit note for 110.00 plus tax (920.00 − 810.00), or a corrected invoice. Without matching, the buyer would have overpaid by 110.00 plus tax.

Related documents in the purchasing cycle

Purchase orders and invoices sit within a family of documents. Knowing which is which avoids confusion when suppliers and buyers use different terms.

Document	Created by	Purpose
Purchase requisition	Buyer's employee	Internal request to buy, before a PO
Request for quotation (RFQ)	Buyer	Asks suppliers to price a requirement
Quote or quotation	Seller	Offers a price and terms
Purchase order	Buyer	Orders goods or services from a chosen supplier
Sales order	Seller	Seller's internal record of the customer's order
Order acknowledgement	Seller	Confirms acceptance of the PO, sometimes with changes
Delivery note or packing slip	Seller	Lists goods shipped, travels with the delivery
Goods received note	Buyer	Records what actually arrived and its condition
Invoice	Seller	Requests payment
Credit note	Seller	Reduces or cancels an invoice
Remittance advice	Buyer	Explains what a payment covers
Statement of account	Seller	Lists open invoices and payments

Watch especially for order acknowledgements that change the price or delivery date. If the buyer does not object, the changed terms may become part of the agreement, depending on the law and the parties' terms. AP teams then find invoices that "don't match" a PO that was effectively amended.

Purchase orders for services

POs were designed for goods, where delivery is easy to verify. Services are harder:

• Define deliverables or time limits in the PO, such as "Up to 40 hours of consultancy at 95.00 per hour, March 2026".
• Use a service receipt or approval step instead of a goods received note: the manager confirms the work was done before the invoice is approved.
• Use blanket or framework POs for ongoing services such as cleaning, IT support or legal work, with a monthly cap.
• Watch for overruns: invoices exceeding the PO value should be approved explicitly, with the PO amended.

Electronic POs and procurement systems

Many organisations now create POs in procurement or ERP systems and send them electronically, sometimes through supplier portals or networks that also carry the invoice back. The advantages are faster cycles, fewer keying errors and automatic matching, because the invoice can be generated directly from the PO. Some e-invoicing networks support exchanging orders as well as invoices in structured formats, which makes the match between them almost automatic. See e-invoicing explained.

Does a small business need purchase orders?

Not always. A small business where the owner approves every purchase personally gains little from formal POs. POs start to earn their keep when:

• Several people can commit the business to spending.
• Purchases are large or frequent enough that overcharging matters.
• Inventory needs tracking against orders.
• Customers or lenders expect formal controls.
• The business is preparing for audit or investment.

A middle path for growing businesses is a simple PO for purchases above a threshold, created in the accounting software, with everything below the threshold approved on the invoice.

Accounting treatment

Purchase orders

Under normal accrual accounting, issuing a PO does not create a liability; nothing has been received yet. Some organisations track open POs as commitments or encumbrances, particularly in the public sector and in budget control systems, so managers know how much of a budget is already committed. These are memo records rather than ledger liabilities in most commercial accounting.

Goods received but not invoiced

If goods are received before the invoice arrives, accrual accounting usually requires recording a liability for goods received not invoiced (GRNI) at period end, so expenses and inventory are not understated.

Invoices

When an approved supplier invoice is recorded, the buyer records the expense or asset (and recoverable tax where applicable) with a corresponding accounts payable liability. The seller records revenue and a receivable. See accounts payable and accounts payable vs accounts receivable.

Benefits of using purchase orders

• Spending control: approvals happen before money is committed.
• Budget visibility: open POs show committed spend.
• Clear terms: price, quantity and delivery are agreed upfront.
• Faster invoice approval: matched invoices can be approved quickly, sometimes automatically.
• Fraud prevention: invoices without a valid PO stand out.
• Audit trail: every payment traces back to an approved request.

When POs may not be worth it

For very small purchases, utilities, rent, subscriptions and recurring services under contract, many organisations use non-PO invoices with a separate approval route. A sensible policy defines which spending requires a PO, often by amount or category. Requiring POs for everything can slow the business and encourage workarounds such as raising POs after the invoice arrives.

Tips for suppliers: getting paid faster

• Always quote the PO number on the invoice, exactly as issued.
• Invoice exactly what was ordered and delivered: same item descriptions, quantities and prices.
• Send invoices to the address specified on the PO, often an AP mailbox or portal.
• Raise changes before invoicing: if prices or quantities change, ask the buyer to amend the PO first.
• Use one invoice per PO unless the buyer says otherwise.
• Follow up early if you have not received an acknowledgement.

Invoices that do not match POs often go into an exception queue, where they can sit for weeks.

Tips for buyers: a clean PO process

• Define when a PO is required and communicate it to suppliers and staff.
• Issue POs before ordering, not after the invoice ("after-the-fact POs" undermine the control).
• Record receipts promptly so three-way matching can happen.
• Set matching tolerances for small price or quantity differences.
• Close POs once fully invoiced to avoid duplicate payments.
• Review open POs regularly and cancel ones no longer needed.

POs, invoices and the bank statement

The final step in the cycle is the payment appearing on the bank statement. Reconciling the bank account confirms that each payment corresponds to an approved, matched invoice, and spotting payments without invoices is a key fraud control. If you need to review historic payments from PDF statements, a bank statement converter creates a list you can compare with your AP ledger. See how to reconcile a bank statement.

Handling disputes between PO and invoice

When an invoice does not match its PO, both sides lose time. A clear dispute process helps:

1. Identify the type of mismatch: price, quantity, item, tax, missing PO number, duplicate.
2. Check internal records first: was the PO amended? Was a receipt recorded late? Many apparent mismatches are internal timing issues.
3. Contact the supplier with specifics: PO line, invoice line, the difference and the evidence.
4. Agree the fix: a credit note, a corrected invoice, a PO amendment or acceptance of the difference.
5. Record the outcome so the same issue does not recur, for example by updating the supplier's price list in the system.

Set expectations for response times on both sides. A supplier waiting weeks for a reply about a 2% price difference will reasonably be frustrated, and late payment can damage the relationship or trigger late payment charges where they apply.

Measuring how well the process works

Useful indicators for buyers include:

• PO compliance rate: the share of spend covered by a PO raised before the order.
• First-time match rate: invoices that match PO and receipt without intervention.
• Exception ageing: how long mismatched invoices wait for resolution.
• After-the-fact POs: POs raised after the invoice date, a sign the process is being bypassed.
• Duplicate payments recovered: a lagging indicator of weak controls.

For suppliers, the equivalent is the share of invoices paid on time and the number queried. Tracking a handful of these monthly shows where to focus.

Worked example: measuring compliance

Over a quarter, a company receives 1,200 supplier invoices worth 2.4 million. Of these, 930 invoices worth 2.1 million had a PO raised before the order date; 90 invoices worth 120,000 had POs raised after the invoice date; and 180 invoices worth 180,000 were non-PO invoices in categories exempt from the policy. PO compliance by value, excluding exempt categories, is 2.1 million ÷ 2.22 million, about 95%. The 90 after-the-fact POs become the focus: most come from one department buying from a single supplier, which suggests a blanket PO would solve the problem.

Common mistakes

• Invoices without PO numbers, causing delays.
• POs raised after the invoice, defeating the purpose.
• Price changes not reflected in the PO, creating mismatches.
• Partial deliveries invoiced in full.
• Duplicate invoices against the same PO.
• Open POs never closed, allowing extra invoices to be matched.
• Confusing PO acceptance with payment obligation: payment is due on the invoice, according to terms, after delivery.

Frequently asked questions

What is the main difference between a purchase order and an invoice?

A purchase order is created by the buyer before the purchase to order and authorize it; an invoice is created by the seller after supplying the goods or services to request payment.

Which comes first, a purchase order or an invoice?

The purchase order comes first. The buyer issues it, the supplier delivers, and then the supplier issues an invoice that references the PO.

Is a purchase order legally binding?

Once accepted by the supplier, a PO generally forms part of a binding contract, subject to the terms agreed and the applicable law. Before acceptance, it is usually an offer that can be withdrawn or amended.

Can I pay an invoice without a purchase order?

Many organisations pay certain invoices without POs, such as utilities, rent or small purchases, through a separate approval process. Where your policy requires a PO, invoices without one should be investigated before payment.

What happens if an invoice doesn't match the purchase order?

The buyer's AP team usually places the invoice on hold and contacts the supplier or the internal requester. The fix may be a credit note, a corrected invoice or an amendment to the PO. Small differences within an agreed tolerance may be approved automatically.

Does a purchase order create a liability in accounting?

Usually not. A PO is a commitment. A liability normally arises when goods or services are received, and is recorded as accounts payable when the invoice is approved, or as an accrual if goods are received before the invoice.

Summary

The purchase order is the buyer's authorization and order; the invoice is the seller's request for payment. Used together, with goods receipts and matching, they control spending, prevent overpayments and speed up approval. Suppliers get paid faster when invoices mirror POs exactly; buyers stay in control when POs are issued before ordering and closed when complete.

When you need to check payments against POs and invoices from PDF bank statements, StatementPilot converts statements into spreadsheets with every transaction checked against the balances.