Purchase order vs invoice
Updated August 2026 · 5 minute read
A purchase order comes from the buyer and authorizes a purchase before it happens ; an invoice comes from the seller and requests payment after delivery. When the invoice arrives, its quantities and prices should repeat what the purchase order agreed, and any gap between the two is a question to settle before paying.
What is the difference between a purchase order and an invoice?
The two documents describe the same goods and often the same numbers, which is exactly why they get confused. What separates them is who wrote them, and when.
| Question | Purchase order | Invoice |
|---|---|---|
| Who issues it | The buyer | The seller |
| When | Before the sale, to authorize it | After delivery, to collect on it |
| What it says | What the buyer agrees to purchase, at what price, delivered where and when | What the seller delivered and what is now owed, with terms and a due date |
| What it commits | An offer to buy, binding once the seller accepts it | A demand for payment against that agreement |
| Its reference number | The PO number, quoted back on the invoice | The invoice number, unique per bill |
| Ledger effect | None yet; it is a commitment, not a transaction | A payable on the buyer's books, a receivable on the seller's |
How does a purchase order become an invoice?
The two documents are stages of one transaction, in a fixed order.
-
The buyer issues the purchase order
Someone with authority decides to spend, and the PO records the decision: items, quantities, agreed prices, delivery terms, and a PO number.
-
The seller accepts and fulfills
Acceptance turns the offer into a contract on the PO's terms. The seller ships the goods or performs the service.
-
The seller invoices against the PO
The invoice bills what was delivered, quotes the PO number, and adds what the PO could not know: the invoice number, the invoice date, and the due date.
-
The buyer matches before paying
Accounts payable compares the invoice to the PO, and to the receiving record if there is one. Agreement clears it for approval; a gap holds it.
-
Payment closes both
The payment settles the invoice, the payable comes off the books, and the PO is marked fully billed so nothing else can bill against it.
Why should the invoice match the purchase order?
The PO number printed on an invoice is the matching key: it points every bill back to the authorization it claims. Insisting that vendors quote it is one of the cheapest AP controls there is, because it makes the comparison mechanical instead of archaeological.
When the comparison fails, the gap has a meaning. A higher unit price is a price increase nobody signed off on. A larger quantity is over-delivery or over-billing. A line the PO never mentioned is an unordered extra. An invoice with no PO behind it at all is either an honest gap in your process or exactly what invoice fraud looks like. None of these are reasons to panic, but all of them are reasons to hold the bill and ask, and none of them surface if the invoice is simply keyed in and paid.
Can a small business skip purchase orders?
Plenty do, and for some spending it is the right call: utilities, subscriptions, and small repeat purchases carry an implied agreement, and writing a PO for each would be ceremony. When one owner personally approves every purchase, the PO's approval function is redundant with the owner's memory, for as long as that memory is the only one that matters.
What skipping costs shows up later. Without a PO there is no written record of the agreed price, so when the invoice arrives the vendor's numbers are the only numbers, and drift goes unnoticed by construction. There is no authorization trail for an accountant, an auditor, or a buyer of the business to inspect, and matching collapses from three documents to one. The usual middle path is a threshold: above a chosen amount, no PO means no payment.
How does LedgerBox pair the two?
LedgerBox reads purchase orders and invoices as distinct document types, each with its own fields, the arithmetic checked against the printed totals and uncertain values flagged for review. Upload both sides and Payables lines each vendor invoice up with its purchase order and the bank transaction that paid it, then states a verdict on whether they agree.
Matched invoices can be approved in bulk; the mismatches become the short list worth a human's attention. The queue exports to CSV, and on higher plans approved documents post to QuickBooks Online as balanced journal entries behind a duplicate guard. Details by plan are on the pricing page.
Questions people ask
Is a purchase order the same as an invoice?
No. The purchase order is the buyer authorizing a purchase in advance; the invoice is the seller billing for it afterward. Same transaction, opposite authors, opposite directions.
Is a purchase order legally binding?
By itself it is an offer. Once the seller accepts it, by confirming it or by fulfilling it, its terms generally bind both sides, which is why the prices on it matter.
What is a PO number and why does it appear on invoices?
It is the identifier the buyer assigned to the purchase order, repeated on the invoice so the bill can be traced to its authorization. AP teams use it as the matching key, and many refuse invoices that omit it.
Can an invoice exist without a purchase order?
Yes, and for utilities, subscriptions, and casual purchases it routinely does. The cost is that nothing independent states the agreed price, so the invoice can only be checked against memory.
Which comes first, the purchase order or the invoice?
The purchase order. It authorizes the spend before goods move; the invoice follows delivery. If an invoice shows up with no PO before it, the purchase was never formally authorized, which is what a no-PO-no-pay policy exists to catch.
Do purchase orders appear in the general ledger?
Not as transactions. A PO is a commitment, tracked as an open order rather than a journal entry; the ledger entry arrives with the invoice, as a payable.
Where to go next
- Purchase order data extraction The fields LedgerBox reads off a PO: lines, quantities, agreed prices.
- Invoice data extraction The seller's side of the pair, extracted the same way.
- Three-way matching in accounts payable What checking PO, invoice, and payment together catches.
- Invoice vs receipt The other commonly confused pair, on the payment side.
- Accounts payable automation for small business Where the PO-to-invoice match sits in the full AP process.