Purchase order (PO): definition and how it works in accounts payable

What it means

A Purchase order is the buyer's written commitment to buy stated goods or services from a supplier at stated prices and quantities. It is not the supplier's bill. The supplier issues an invoice, and AP uses the PO as the reference for checking what was ordered, what was received and what should be paid.

POs take different forms. A one-off PO covers a single purchase. A blanket or standing PO covers repeated purchases over a period up to a limit. The period, limit and approval rules vary by company and contract, so AP should read the PO rather than assume that any open PO authorizes any invoice from that supplier.

An invoice without a PO is a non-PO invoice and needs a different approval route. That route may be manager sign-off, budget owner approval or another control set by the company. It is not a reason to skip the control question: should this supplier be paid, for this amount, at this time? When goods arrive before the invoice, the receipt sits against the PO and is often tracked as goods received not invoiced.

How it works in practice

The buying team raises and approves the PO before the supplier performs. The supplier receives the PO and should quote its number on the invoice and on delivery documents. AP then has a common reference for the order, the receipt and the bill.

When the invoice arrives, AP captures the PO number and checks the invoice against the PO. The PO number on the invoice lets AP match it automatically. In a three-way match, AP also checks the receipt so that payment follows evidence that the goods or services were actually received. The right matching method depends on what is bought, the contract and company policy.

A blanket or standing PO needs closer attention than a one-off PO. The supplier may invoice several times over the period, and AP must know the limit and the period and escalate when the limit is reached. A blanket PO is not a standing approval to pay every invoice that cites it.

For a non-PO invoice, AP should follow the company's separate approval route and keep the evidence with the invoice. The route varies by company and contract, so AP should not treat an email approval as enough unless that is the documented process.

Where it goes wrong

A common mistake is paying an invoice that has no PO number because the approval email looks complete. AP may later find that a PO exists but was never linked to the invoice. Before treating an invoice as non-PO, check whether a PO exists for that supplier and purchase, and ask the supplier for the PO number when the invoice should have one.

Another mistake is matching to the wrong PO. The invoice may quote a PO number that belongs to another supplier, another purchase or a PO that has already been closed. A closed PO is not authority for a new purchase. AP should confirm that the PO is open, belongs to the supplier and covers the items and quantities on the invoice.

Price and quantity differences cause trouble when AP pays first and investigates later. A purchase price variance is a signal to check the PO, the receipt and the contract before payment. A PO raised after the work was done is another warning sign: a retroactive PO may clear the invoice but it destroys the control the PO was meant to provide.

Blanket POs fail when AP does not track the limit or period. A supplier may invoice repeatedly against one blanket PO until the limit is exceeded. Duplicate invoices against the same PO also happen when AP checks the PO but not the receipt and prior invoices. The catch is to compare the invoice with the PO, the receipt and the payment history before releasing funds.

What to check

Before paying, confirm that a valid PO exists, that it is approved and open, and that it names the same supplier as the invoice. If no PO exists, route the invoice through the non-PO approval process instead of forcing it against an unrelated PO. If a PO exists but is closed, treat the invoice as a new commitment and route it for approval.

Match the invoice price and quantity to the PO and to evidence of receipt. In a three-way match, the receipt is part of the payment evidence. If the invoice shows a different quantity, price or item, resolve the difference with the buyer and supplier before payment. Country, company and contract rules affect what evidence is acceptable, so follow the documented policy for the purchase.

For blanket POs, check the period, the limit and the invoices already charged against it. Confirm that the current invoice still falls within the authorized period and limit. Do not rely on the supplier's statement of the remaining balance; check the PO and the internal records.

The PO, receipt, invoice and approval should tell one story. Keep the PO number on the invoice record, keep the receipt evidence and keep the approval that applies to the route used. When those documents disagree, stop and resolve the difference rather than paying on the strength of the invoice alone.

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