A purchase order and an invoice bracket the same transaction from opposite ends. The purchase order comes first: the buyer issues it to say "we want this, at this price — go ahead." The invoice comes last: the seller issues it to say "we delivered this, at that price — now pay." Between them sits the actual work or delivery.
The two documents are easy to confuse because they often list nearly identical information — the same items, quantities and prices. What differs is who creates them, when, and what each one commits its sender to. Getting that wrong is more than a filing error: buyers can end up paying for things nobody ordered, and sellers can end up doing work no one agreed to pay for.
What a purchase order is
A purchase order (PO) is a document the buyer sends to a supplier to request and authorise a purchase. It states what the buyer wants to buy, in what quantity, at what agreed price, and usually where and when it should be delivered. Each PO carries a unique PO number, which becomes the reference that every later document in the transaction points back to.
Once the supplier accepts it, a purchase order is generally treated as a binding commitment to buy on those terms. That is its real function inside larger companies: nothing gets bought without an approved PO, which is how finance teams control spending before it happens rather than discovering it afterwards.
What an invoice is
An invoice is a document the seller sends to the buyer after delivering goods or completing work, formally requesting payment. It lists what was actually delivered, the amount now owed, any tax, the payment terms, and how to pay. Where a PO says "please supply", an invoice says "please pay".
If the purchase started with a PO, the invoice should quote that PO number. That single reference is what lets the buyer’s accounts payable team match the request for money against an authorised purchase — and it is the most common reason supplier invoices get paid promptly instead of sitting in a query queue.
The differences at a glance
The quickest way to keep the two straight is to remember that each document answers a different question: the PO answers "what did we agree to buy?", the invoice answers "what do we now owe?"
| Purchase order | Invoice | |
|---|---|---|
| Issued by | Buyer | Seller |
| Issued when | Before goods ship or work starts | After delivery or completion |
| Purpose | Authorise and record the order | Request payment for what was delivered |
| Commits | Buyer to purchase on the stated terms | Buyer to pay within the stated terms |
| Key reference | PO number (newly created) | Invoice number, plus the PO number it fulfils |
| Handled by | Procurement / the person buying | Accounts payable (buyer side), accounts receivable (seller side) |
How the two documents work together
In a full procurement flow the PO and invoice are two ends of a paper trail that verifies itself. The buyer raises a PO; the supplier delivers and sends an invoice quoting the PO number; the buyer’s accounts payable team then matches the invoice against the PO — and, in stricter setups, against a goods received note as well. Invoice, order and delivery all have to agree before payment is released.
That check is called two-way matching (invoice against PO) or three-way matching (invoice against PO and delivery record). It exists to catch exactly the failure modes that cost businesses money: invoices for things never ordered, prices that quietly grew between order and billing, and quantities billed but never delivered.
- Buyer raises and approves a purchase order, and sends it to the supplier.
- Supplier accepts the PO, then delivers the goods or performs the work.
- Supplier issues an invoice that quotes the PO number.
- Buyer matches invoice ↔ PO (± delivery record) and pays within the agreed terms.
Does a small business need purchase orders?
If you are a freelancer or a small service business, you will meet POs mostly from the buyer’s side of the table: a larger client says "we’ll raise a PO for this" and expects your invoice to quote their PO number. Do that, every time — an invoice that arrives without the PO reference usually cannot be matched, and unmatched invoices are where payment delays are born.
Issuing your own POs matters once you regularly buy materials or subcontract work. A plumber ordering fixtures for a job, or an agency booking a freelancer, gains a record of what was agreed before the supplier’s invoice arrives — which turns "is this bill right?" from a memory test into a document comparison. For pure service work with nothing purchased, a signed quote or contract usually plays the PO’s role and a separate document adds little.
Common Mistakes
Treating the PO as the bill
A purchase order requests goods; it never requests money. Paying "off the PO" means paying before anything was delivered or checked. Payment should follow the invoice, after matching.
Invoicing without the PO number
If the client gave you a PO, an invoice that does not quote its number often cannot be processed at all. It stalls in accounts payable while someone hunts for the order it belongs to.
Letting the invoice silently differ from the PO
If the scope or price changed after the PO was raised, get the PO amended before invoicing. An invoice that disagrees with its PO fails matching and triggers a query cycle — the slowest route to payment there is.
Skipping POs for subcontracted costs
Buying materials or booking subcontractors on a phone call leaves you with no record of the agreed price when the invoice lands. A one-line PO — even an email with a number — gives you something to match against.
Frequently Asked Questions
Is a purchase order legally binding?
Generally, once the supplier accepts it, a purchase order is treated as a binding agreement to buy on the stated terms — though the details depend on the contract law that governs the transaction. Before acceptance it is an offer, not a contract.
Can an invoice exist without a purchase order?
Yes, and for most freelancers and small businesses it usually does. POs are a control layer that larger organisations add; a quote, contract or plain agreement can fill the same role. If the buyer did issue a PO, though, the invoice should always reference it.
Which comes first, the invoice or the purchase order?
The purchase order. It authorises the purchase before goods ship or work starts; the invoice follows delivery and requests payment. If an invoice arrives before any order was agreed, that is a red flag worth querying.
What is PO matching?
The buyer-side check that compares an incoming invoice against the purchase order it claims to fulfil (two-way matching), and often against the delivery record too (three-way matching). Quantities, prices and items must agree before the invoice is approved for payment.
Sources & Further Reading
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