Purchase Order vs Invoice: Differences, Which Comes First, and the Three Way Match

A purchase order is issued by the buyer before anything is delivered and authorizes the purchase. An invoice is issued by the seller after delivery and requests payment for it. The purchase order comes first, states what the buyer agreed to buy and at what price, and the invoice is checked against it before the money goes out. One is an offer to buy, the other is a demand to be paid.

Both documents describe the same transaction, which is why they get confused, and both carry the same line items, quantities, and prices. The difference that matters in practice is direction and timing. The purchase order flows from buyer to seller and commits the buyer to spend. The invoice flows from seller to buyer and asks for the money. When accounts payable checks one against the other and the numbers agree, the invoice gets paid. When they do not agree, somebody has to explain why, and that conversation is easier when the paperwork was right the first time.

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

The purchase order is the buyer's written authorization to purchase specific goods or services at an agreed price. The invoice is the seller's request for payment after those goods or services have been supplied. The PO is created first and sets the terms, the invoice is created last and references those terms, usually by quoting the PO number.

Three practical consequences follow from that. First, the PO is what a budget holder approves, so it is the control point for spending. Second, the invoice is what triggers the payment clock, so the payment terms on it determine when cash leaves. Third, if the two documents disagree, the PO is normally the reference point, because it records what was actually authorized.

Does a purchase order or invoice come first?

The purchase order comes first. The sequence in a standard business purchase is quote, purchase order, delivery, invoice, payment, receipt. The buyer issues the PO to authorize the spend, the supplier delivers against it, and only then does the supplier invoice. An invoice that arrives with no purchase order behind it is a sign that somebody bought something outside the process.

There is one common exception. Low value or recurring purchases are often handled without a PO at all, on a standing agreement or a company card, and the invoice is the only document in the chain. That is a deliberate choice to trade control for speed on small amounts, and most finance teams set a threshold below which no PO is required.

Are invoices and purchase orders the same thing?

No. They are issued by opposite parties, at opposite ends of the transaction, and they do different jobs. A purchase order commits the buyer to a purchase. An invoice commits nobody to anything new, it simply asks for payment already owed under an agreement that exists.

The legal difference is sharper than the paperwork suggests. Under the Uniform Commercial Code, which governs the sale of goods in every US state, a purchase order is generally treated as an offer. The contract forms when the seller accepts it, whether by sending back a sales order, by acknowledging it in writing, or by shipping the goods. An invoice is not an offer and it is not a contract. It is a statement of what is owed under a contract that already formed.

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

A purchase order authorizes a purchase before it happens, an invoice requests payment after delivery, and a receipt proves the payment was made. They are three points on the same timeline, and each one is the evidence for a different question: what did we agree to buy, what do we owe, and what did we pay.

Add the quote and the sales order and you have the full set of documents most US businesses exchange on a single transaction:

Two of these get mixed up more than the rest. A sales order is not a second purchase order, it is the seller's confirmation of yours. And a receipt is not a small invoice, it is proof that an invoice was settled. If a supplier sends you something labelled a receipt while you still owe the money, it is an invoice with the wrong heading on it.

Where does a quote fit between a purchase order and an invoice?

A quote comes before the purchase order. The seller quotes a price, the buyer decides to proceed and issues a purchase order that usually references the quote number, and the invoice at the end should match both. A quote is not binding on the buyer and normally expires, which is why the price on an old quote is one of the most common reasons an invoice fails to match the PO.

If your quotes routinely turn into signed agreements rather than into POs, you are running a different process, and the signature is what forms the contract instead of the PO. That is normal in services work. It still helps to raise a PO afterwards so the commitment lands in the budget rather than surfacing when the invoice does.

What happens when the purchase order and the invoice do not match?

The invoice goes on hold until the discrepancy is explained. Most accounts payable teams run a three way match, comparing the purchase order, the goods receipt confirming what actually arrived, and the invoice. If all three agree, the invoice is approved for payment automatically. If any one of them disagrees, a human has to resolve it before the payment runs.

The three way match is worth understanding even if you are the supplier rather than the buyer, because it explains why a correct invoice can still sit unpaid for weeks. Here are the mismatches that cause it and how each one gets cleared:

Suppliers can remove most of this friction unilaterally. Put the PO number on the invoice, invoice only what was actually delivered, and invoice each partial shipment separately rather than sending one combined invoice at the end. Buyers with more than a handful of orders a month usually stop tracking this in a spreadsheet and move it into purchase order management software so the approval, the receipt, and the invoice all reference the same record.

Purchase order date vs invoice date: which one matters?

The invoice date starts the payment clock, so net 30 is counted from the invoice date rather than the purchase order date. The purchase order date matters for a different reason: it is when the buyer committed the budget, and it is the date auditors look at when they ask whether the spend was approved before it was incurred.

The gap between the two dates is a useful diagnostic. A long gap usually means slow fulfilment or slow invoicing. A negative gap, an invoice dated before the purchase order, means somebody bought first and raised the paperwork afterwards, which is exactly the pattern internal controls are designed to catch. For revenue recognition, neither date is decisive on its own, because revenue is recognized when the obligation to the customer is satisfied rather than when a document is dated.

Do purchase orders need to be signed?

Not always. A purchase order issued under an existing supply agreement usually needs no signature at all, because the master agreement already sets the terms and the PO simply calls off against it. Signatures matter when the PO itself carries the terms, when the amount clears an internal approval threshold, or when the supplier wants written acceptance before committing materials.

When a signature is needed, an electronic one is fully valid. The federal ESIGN Act and the state UETA statutes make an electronic signature on a commercial purchase order as enforceable as ink, and the audit trail that comes with it, showing who signed, from what email address, and when, is stronger evidence than a scanned page. The longer version of that argument is in our guide on whether a purchase order can be signed electronically, and if you need to get one signed today you can send the document for signature and have it back the same day.

Purchase order financing vs invoice financing

Purchase order financing funds the cost of fulfilling an order you have received but cannot yet afford to produce, using the customer's purchase order as the basis for the advance. Invoice financing, sometimes called factoring, advances cash against invoices you have already issued and are waiting to be paid on. The first solves a production problem, the second solves a collection problem.

The distinction follows directly from the difference between the two documents. A purchase order proves somebody intends to buy. An invoice proves you already delivered and are owed. Lenders price those two risks differently, which is why PO financing generally costs more: nothing has been delivered yet, so more can still go wrong.

Purchase order vs commercial invoice

A commercial invoice is a specific kind of invoice used for international shipments, and customs authorities rely on it to assess duties and clear the goods. It carries details a domestic invoice does not need, including the country of origin, the harmonized tariff code, and the incoterms that decide who pays freight and bears risk in transit.

The purchase order still comes first and still authorizes the purchase. The change is that the commercial invoice has an audience beyond the buyer's accounts payable team. If it does not agree with the purchase order and the packing list, the shipment can be held at the border, which is a more expensive kind of mismatch than a payment delay.

How long should you keep purchase orders and invoices?

Keep both for at least as long as the tax authority can question the return they support. The IRS generally expects records that substantiate income or deductions to be kept for three years from the date the return was filed, with longer periods in specific circumstances, so most US businesses keep purchase orders and invoices for seven years as a practical policy that also covers state requirements and contract limitation periods.

Keep them together. An invoice on its own proves you paid something. An invoice with the purchase order behind it proves you paid something that was authorized, at the price that was agreed, for goods somebody confirmed had arrived. That pairing is what an auditor is actually looking for, and it is the reason the two documents belong in the same file rather than in two systems that never speak to each other.

The short version

The buyer writes the purchase order and it comes first. The seller writes the invoice and it comes last. Everything between them, the quote, the sales order, the delivery, the goods receipt, exists so that when the invoice finally arrives, somebody can check it against what was agreed and pay it without a phone call. Get the purchase order right and the invoice mostly takes care of itself.