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Purchase Order vs. Invoice: Who Issues What, and When

Both list items and prices, but they come from opposite sides of a transaction, at different points in the process. Here is how they fit together, with an example.

Evrenus Fırat Polat · ·

A purchase order and an invoice can look almost identical at a glance. Both list what is being bought, the quantities, and the price. The real difference is not in the layout. It is in who issues each one, and when.

A purchase order comes from the buyer, before the sale

A purchase order is issued by the client, the buyer, as their own internal authorization to make a purchase. It is sent to you before or as work begins. It represents their commitment: "we're ordering this, at this price, and it's been approved on our end."

An invoice comes from the seller, after the sale

An invoice is issued by you, the seller, after goods or services have been delivered, or at an agreed milestone. It represents your request for payment: "here's what we delivered, and here's what's owed."

Side-by-side

Purchase order Invoice
Issued by Buyer Seller
When Before delivery After delivery or milestone
Meaning "We will buy this" "Please pay for this"
Creates Buyer's commitment Seller's receivable and buyer's payable
Reference numbers PO number Invoice number, citing PO number

Why the distinction matters in practice

Confusing the two, or treating a PO as a payment claim without a corresponding invoice, causes real problems. A PO alone does not get you paid. It is an authorization on the buyer's side, not a payment claim on yours. You still need to send your own invoice, referencing the PO number, before payment can be processed in most companies' systems.

How they work together

The typical sequence:

  1. You send a quote.
  2. The client accepts and issues a PO, their internal go-ahead.
  3. You confirm the order, sometimes with a sales order.
  4. Work happens or goods ship, with a packing slip or delivery note.
  5. You invoice, citing the PO number, so the client's accounts-payable team can match your invoice to their approved order.
  6. The client pays and you issue a receipt if needed.

Skipping the reference, or invoicing for a different amount than the PO authorized without flagging the change, is one of the most common reasons a correct, honest invoice still gets stuck in review.

Worked example: matching an invoice to a PO

A client issues PO-8802 for 40 hours of consulting at $95 an hour, with a cap of $3,800.

  • PO value: 40 × $95 = $3,800.

You deliver 44 hours. Your invoice would be 44 × $95 = $4,180, which exceeds the PO by $380. If you send it as is, the client's system may reject it because it does not match.

Better approach:

  1. Before hour 41, tell the client you will exceed the PO.
  2. Ask them to issue a revised PO or an addendum, say PO-8802-A, for 4 more hours: 4 × $95 = $380.
  3. Invoice $3,800 against PO-8802 and $380 against PO-8802-A, or one invoice citing both.

If no revision arrives, bill $3,800 now and handle the extra $380 separately once approved. Sales tax or VAT, if it applies, is added on top according to local rules.

Checklist before invoicing against a PO

  1. Is the PO number on the invoice?
  2. Do the items, quantities, and unit prices match the PO?
  3. Is the total within the PO limit?
  4. Is the billing address the one on the PO?
  5. Did you include the delivery reference, if goods were shipped?
  6. Did you send it to the AP contact or portal named by the client?

Use the invoice generator and add the PO number in the reference field. See what to include on an invoice.

If a client doesn't use purchase orders

Plenty of businesses, especially smaller ones, do not issue POs at all. An accepted quote or estimate serves as the equivalent commitment, and you invoice directly against that. Neither approach is more correct. It depends entirely on how formal the client's own procurement process is.

Common mistakes

  • Leaving off the PO number. The invoice gets bounced.
  • Invoicing before the PO exists. Some buyers will not pay unless a PO came first.
  • Exceeding the PO without approval. The extra amount can be refused.
  • Issuing a PO yourself and calling it an invoice. Different documents, different roles. If you buy from vendors, use the purchase order generator.
  • Not tracking PO balances. Keep a note of what remains on each PO, especially for multi-delivery orders.

For slow approvals, see reducing late payments, since a missing PO number is a common hidden cause.

FAQ

Can I invoice before I receive a PO?

You can, but the client may hold payment until a PO is in place. Ask for the PO before starting.

Is a PO a legal contract?

It may form part of a contract once accepted, but terms differ by place. Check the terms printed on it and, for large deals, ask a lawyer.

Who keeps the PO?

Both sides do. The buyer uses it for approval, and the seller uses it to support the invoice.

What if the invoice differs from the PO?

Explain the difference clearly on the invoice and tell the client before sending. A revised PO is the cleanest fix.

Same information, opposite direction. A PO says "we're going to buy this." An invoice says "we sold this, please pay." Keeping that direction straight keeps both documents doing their job.

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