What is a purchase order and what is an invoice?
A purchase order is a document the buyer sends to a supplier to order goods or services. It can state the scope, quantity, agreed price, delivery instructions and terms. Whether it becomes a binding contract on issue, acceptance, performance or another event depends on the governing law and the parties' agreement. Conflicting supplier and buyer terms can also matter.
An invoice is normally issued by the seller to state what is being billed and when payment is due. It records a claim under the underlying order or contract; the invoice itself does not necessarily create that obligation. Its timing, numbering and tax content depend on the transaction and local rules. Finance teams often compare it with the PO and delivery or acceptance record before approving payment.
Who issues each document, and when?
The usual direction is buyer to seller for a PO and seller to buyer for an invoice. The stage is less rigid. A supplier might invoice for a deposit before work, by milestones during a project, or after delivery. Self-billing and marketplace arrangements can change who prepares the invoice.
Use the document's purpose and terms, not its layout alone, to identify it.
| Aspect | Purchase order (PO) | Invoice |
|---|---|---|
| Usual issuer | Buyer | Seller, subject to permitted self-billing arrangements |
| Usual timing | Before supply, to place an order | At the agreed or legally required invoicing point |
| Purpose | Authorize and describe a purchase | State what is billed and when it is due |
| Main reference | PO number assigned by the buyer | Invoice identifier required by the applicable rules |
| Operational effect | Supports approval and fulfilment | Supports payment approval and accounting |
| Tax status | Not normally a tax invoice | Valid tax evidence only when it meets the applicable requirements |
What information goes on a PO versus an invoice?
When the documents concern the same ordered supply, their fields often overlap: the parties, line items, quantities, agreed prices and tax treatment. The differences are in the identifying references and the intent. A purchase order leads with the PO number and delivery instructions; an invoice leads with the invoice number, an issue date and a due date, and the payment details. The two are deliberately built to line up so they can be matched. FreeBillGen can prepare the invoice draft, but it has no dedicated PO-number field; if your buyer accepts it, include the reference clearly in an available line description or note.
- Document title
- Label a PO "Purchase order" and an invoice "Invoice" so their roles are clear.
- Reference numbers
- A PO carries the buyer's reference. An invoice carries its own required identifier and often quotes the PO reference.
- Buyer and seller details
- Both commonly identify the parties, but required names, addresses and tax identifiers vary.
- Line items and prices
- Describe the scope, quantity and price clearly enough to match agreed items and approved changes.
- Dates
- A PO can show order and requested delivery dates. An invoice can show issue, supply and due dates.
- Tax
- A PO can estimate tax. An invoice must use the actual tax treatment and meet local tax-invoice rules where relevant.
- Terms and instructions
- A PO often states delivery and acceptance terms. An invoice normally states payment terms and a verified payment method.
How does PO-to-invoice matching work?
PO-to-invoice matching is an accounts-payable control. The team compares the supplier's invoice with approved purchasing records before payment. A mismatch can be routed for review rather than treated as proof that either document is wrong.
Three-way matching compares the purchase order, a goods-received or service-acceptance record, and the invoice. Two-way matching compares the PO and invoice. Systems can allow agreed tolerances for price, quantity, freight, tax or partial deliveries, so an exact character-for-character match is not always required.
Matching can catch duplicate invoices, unapproved quantities and price differences. Quote the buyer's PO reference when their instructions require it, and document authorized changes rather than altering an invoice just to force a match.
- Two-way match
- Compares the purchase order with the invoice (price and quantity). Common for services with no physical goods to receive.
- Three-way match
- Compares the purchase order, a receipt or acceptance record, and the invoice. Common for controlled purchases.
- Apply tolerances
- Review differences against the buyer's approved tolerance and exception rules instead of assuming every mismatch blocks payment.
- Quote the PO number
- Reference the buyer's PO when required so accounts payable can find the approved order.
When do you actually need a purchase order?
Not every sale uses a purchase order. Many small businesses, freelancers and retail transactions work from an accepted quote or contract. Larger organizations often require a PO because it records budget approval and supports an audit trail.
If the buyer uses a PO system, get the approved PO and invoicing instructions before starting where practical. An accepted quote can record similar commercial details, but it is not automatically equivalent to a PO in the buyer's controls or under the governing contract.
Purchase order and invoice questions
What is the main difference between a purchase order and an invoice?
A purchase order is usually the buyer's documented order. An invoice is usually the seller's statement of what is billed and due. They can be matched, but their timing and legal effect depend on the contract, tax rules and procurement process.
Does a purchase order come before or after an invoice?
Usually before. The buyer commonly issues a PO before supply, then the seller invoices at the agreed or legally required point. Deposits, milestone billing, recurring supplies and self-billing can produce a different sequence.
Can a purchase order be used as an invoice?
Generally no. A PO authorizes or describes the purchase; it is not normally the seller's bill or valid VAT evidence. The agreement may still make payment due without a conventional invoice, while a tax invoice is valid only if it meets the applicable requirements.
What is three-way matching?
Three-way matching compares the purchase order, a goods-received or service-acceptance record, and the invoice before approval. The buyer can apply documented tolerances and route differences for review. It is a control, not a rule that every character must match.
Do I have to put the PO number on my invoice?
Include it when the contract or buyer's invoicing instructions require it. Missing or incorrect references can delay approval or lead to rejection under that process. Ask for the approved reference and do not invent one.
Do small businesses and freelancers need purchase orders?
Not necessarily. Many use an accepted quote or contract instead. Use a PO when the buyer's process or the agreement requires one, especially where spending approval, recurring orders or a formal audit trail matters.