Purchase Order vs Invoice: The Complete Guide

A purchase order and an invoice describe the same transaction from opposite ends. The buyer issues the purchase order before anything is supplied, setting out what has been agreed. The supplier issues the invoice afterwards, asking to be paid for what was delivered. Understanding how the two documents relate, and how the PO number binds them together, is the foundation of every reliable accounts payable control. This guide explains both documents, compares them side by side, and shows what to do when they disagree.

Two documents, one transaction

Almost every business purchase produces two pieces of paperwork that describe the same event. The first is written by the party doing the buying and looks forward: this is what we want, in these quantities, at these prices, delivered by this date. The second is written by the party doing the selling and looks backwards: this is what we supplied, so this is what you owe us.

People often treat the two as interchangeable because they carry similar line items, similar totals and often the same supplier name. They are not interchangeable. They are issued by different parties, at different moments, for entirely different purposes, and they carry different legal weight. Confusing them causes real damage: duplicate payments, unauthorised spend, and accruals that misstate what the business actually owes at period end.

The healthiest way to think about it is that the purchase order is the promise and the invoice is the claim. The value of running both properly comes from comparing them. When the claim matches the promise, payment is routine. When it does not, you have found something worth investigating before the money leaves the account rather than after.

What a purchase order is and who issues it

A purchase order is a commercial document created by the buyer and sent to the supplier to authorise a purchase. It names the items or services, the quantities, the agreed unit prices, the delivery address and date, and the payment terms. Crucially, it carries a unique PO number that will follow the transaction to its conclusion.

The PO is issued before any goods move. Internally it usually follows a requisition and an approval step, so by the time it reaches the supplier it represents budget that has been checked and authority that has been granted. On its own it is an offer; once the supplier accepts it, whether by confirmation or by fulfilling it, the terms on the PO become the contract governing that purchase.

Because the buyer writes it, the PO is where commercial control lives. It fixes the price before the supplier has any leverage, it caps the quantity, and it records the terms in a form both sides can point to later. Our full purchase order guide covers the document structure, the common PO types and the approval workflow in more depth.

What an invoice is and who issues it

An invoice is a document created by the supplier and sent to the buyer to request payment for goods or services already delivered. It lists what was supplied, the amounts due, any tax applied, the payment terms and the due date, and it carries the supplier's own sequential invoice number for their records.

Timing is what separates it from the PO. The invoice is raised after fulfilment, which means it reflects what actually happened rather than what was planned. If the supplier shipped nine of ten items because one was out of stock, a correct invoice bills for nine. If a freight charge applied that nobody anticipated, it appears here first. The invoice is therefore the document where reality first meets the original agreement.

The invoice also has a tax and accounting role the PO does not. It is the instrument that triggers a liability in the buyer's ledger, supports input tax recovery in most jurisdictions, and starts the payment clock. Our invoice guide covers the mandatory fields, tax treatment and numbering rules in full.

Purchase order vs invoice, side by side

Setting the two documents against each other makes the division of labour obvious. Read down the columns and you can see why one cannot replace the other.

The pattern is consistent: the PO is forward looking and buyer controlled, the invoice is backward looking and supplier controlled. Neither is more important. The control comes from holding both and insisting they agree.

How the PO number ties the two together

The single most useful field on either document is the PO number. The buyer generates it when the order is raised, prints it on the PO sent to the supplier, and expects to see it quoted back on the invoice. That one string turns two unrelated documents into a matched pair.

In practice the number does several jobs at once. It tells accounts payable which approved order this bill relates to, so nobody has to guess. It lets the system retrieve the agreed prices and quantities automatically instead of asking a buyer to remember them. It links the goods received note to the same transaction. And it prevents the same order being paid twice, because a second invoice quoting a PO that is already fully consumed is flagged immediately.

This is why mature buying organisations enforce a simple rule: no PO number, no payment. It sounds bureaucratic, and suppliers occasionally grumble, but the alternative is an inbox of bills that nobody can tie to an authorised purchase. Sending the PO number to the supplier clearly, and requiring it on their remittance documents, costs nothing and eliminates most matching work before it starts.

What three-way matching checks and why it matters

Three-way matching is the control that makes the PO and invoice relationship worth having. Before an invoice is approved for payment, three documents are compared: the purchase order, which says what was authorised; the goods received note, which says what actually arrived; and the invoice, which says what is being billed. Payment proceeds only when all three agree.

The check itself is narrow and mechanical. Does the invoiced quantity match the received quantity? Does the invoiced unit price match the PO price? Do the line items correspond to lines on the order? Is the total within any tolerance the business has set? Each answer is either yes or a flagged exception, and the exceptions are the only items a human needs to look at.

The reason this matters goes beyond arithmetic. Three-way matching is the primary defence against invoice fraud, because a fabricated bill has no PO to match and no receipt to support it. It is also what makes accounts payable scalable: when the routine invoices clear themselves, the team spends its time on the small proportion that genuinely need judgement. For the mechanics of running this at volume, see our guide to processing PO invoices.

PO-backed and non-PO invoices

Not every invoice that arrives has an order behind it, and the two categories need different handling. A PO-backed invoice can be matched and cleared largely without human involvement. A non-PO invoice has nothing to match against, so approval has to be constructed after the fact.

The practical goal is not to eliminate non-PO invoices entirely, since some categories genuinely do not suit ordering in advance, but to shrink them to a deliberate, named list. Everything outside that list should require a PO. Measuring your PO coverage rate, the share of invoice value that arrives PO-backed, is one of the quickest ways to see how much control you actually have over spend.

What to do when the PO and invoice disagree

Mismatches are normal. Prices move, deliveries split, tax gets calculated differently. What separates a well-run function from a chaotic one is having a defined response for each type of variance rather than letting disputed invoices age quietly in a drawer.

Price variances mean the invoice bills a different unit price to the PO. Check the agreed price first: if the PO is right, the supplier reissues; if a genuine increase was agreed but never recorded, amend the PO so the trail reflects the decision. Quantity variances mean the billed quantity differs from what was received. Partial deliveries are legitimate and should be billed partially, so verify against the goods received note before assuming an error. Tax variances usually come from a wrong rate, a missing tax registration number or a cross-border treatment the buyer did not expect, and they need correcting on the invoice itself because tax cannot normally be reclaimed on a defective document.

Two habits make all of this manageable. Set sensible tolerances, so that a variance of a few pence on freight does not consume an hour of somebody's day, while anything material stops dead. And hold the invoice while the query is open rather than paying and chasing a refund, because recovering money from a supplier is always harder than not sending it.

The record trail auditors expect

When an auditor samples a payment, they are testing whether the money that left the business was authorised, supported and correctly recorded. They do that by walking the documents backwards from the bank entry, and every link in that chain needs to be present and consistent.

• The requisition: evidence that somebody with budget asked for the purchase before it happened.
• The approval: a record of who authorised the spend, at what value, and under which delegation limit.
• The purchase order: the terms sent to the supplier, with its unique number and any subsequent amendments.
• The goods received note: confirmation of what arrived, when, and who accepted it.
• The supplier invoice: the original document, quoting the PO number, with tax stated correctly.
• The match result: proof that the three-way check ran and either passed or was resolved.
• The payment record: the remittance tying the cleared invoice to the actual bank transaction.

Assembling that chain by hand from email threads and shared drives is slow and rarely complete. When requisition, order, receipt, invoice and payment live in one system linked by the PO number, the trail builds itself as work happens and any transaction can be reconstructed in seconds.

That is the flow ProcureWave is designed around: approved requisitions become purchase orders without re-keying, receipts post against those orders, and supplier invoices match automatically so only genuine exceptions reach a person. If you would like to see how your own PO and invoice matching would look, have a chat with our team.

A purchase order and an invoice are not rivals or duplicates. One records what you agreed to buy, the other records what you are being asked to pay, and the discipline of comparing them is what turns a pile of paperwork into genuine financial control. Get the PO number on every invoice, match before you pay, handle variances by rule rather than by mood, and the record trail an auditor wants will already be there when they ask for it.

Frequently asked questions

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

A purchase order is issued by the buyer before goods or services are supplied, and it authorises the purchase on agreed terms. An invoice is issued by the supplier after delivery, and it requests payment. The PO records what was ordered; the invoice bills for what was actually supplied.

Which comes first, the purchase order or the invoice?

The purchase order always comes first. The buyer raises and approves a purchase order, the supplier accepts and fulfils it, and only then does the supplier issue an invoice referencing that PO number.

Does an invoice have to reference a purchase order number?

Not legally, but most buying organisations insist on it. The PO number is the link that lets accounts payable match the bill to the order and the receipt. Invoices arriving without a valid PO number are usually held or returned to the supplier for correction.

What is three-way matching?

Three-way matching compares three documents before payment is released: the purchase order, the goods received note, and the supplier invoice. When quantity and price agree across all three, the invoice is approved automatically. Any disagreement is flagged as an exception for a human to review.

What should you do if the invoice does not match the purchase order?

Hold the invoice rather than paying it, identify whether the variance is in price, quantity or tax, and check which document is wrong. Genuine changes are resolved by amending the PO or issuing a credit note; errors are corrected by the supplier reissuing the invoice.