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In any accounts payable process, two documents do most of the heavy lifting: the purchase order and the invoice. Finance teams deal with both every day. Yet the distinction between them, what each one is, who creates it, and what it commits to, is less clear in practice than it should be.
When the difference is well understood and the workflow connecting the two is properly managed, the AP cycle runs smoothly. When it is not, mismatches cause payment delays, supplier disputes, and the kind of errors that take significant time to investigate and resolve.
This guide covers both documents clearly, how they fit together in the payment workflow, and why the connection between them is one of the most important controls in accounts payable automation.
A purchase order is a document created by the buyer. It is a formal, written commitment to purchase specific goods or services from a supplier, on agreed terms.
The PO is generated before the goods or services are delivered. It commits the buyer to the transaction and gives the supplier a reference point for fulfilling the order and, later, for issuing an invoice.
From the buyer's perspective, the purchase order is the authorisation step. It confirms that the purchase has been approved internally, that the budget exists, and that the supplier is on the approved vendor list. Without a PO, the purchase is informal: there is a verbal or email commitment, but no document that can be matched against a delivery note or an invoice downstream.
From the supplier's perspective, the PO is the instruction to fulfil the order. It tells them what is needed, when, and at what price. It is also the reference they will use when issuing their invoice.
POs are most commonly used when:
Not every purchase generates a PO. Ad-hoc, low-value, or one-off purchases are often processed as non-PO invoices, which require a different verification path and tend to carry higher risk because there is no pre-approved commitment to match against.
A well-formed purchase order should contain enough information for both the buyer and the supplier to understand exactly what is being transacted. At minimum, a PO should include:
Buyer details: Company name, registered address, and VAT number.
Supplier details: Supplier name, address, and the contact the PO is addressed to.
PO number: A unique reference number assigned by the buyer's system. This number connects the PO to the delivery note and the invoice in the matching process. It should appear on all related documents.
Line items: Each product or service being purchased, described specifically enough to confirm receipt and match against the invoice. This includes the item description, quantity, unit price, and the total for each line.
Delivery details: Where the goods should be delivered or the service performed, and by when.
Payment terms: The agreed payment period, such as Net 30 or Net 60, and any applicable early payment discount conditions.
Budget or cost centre reference: The internal code that allocates the spend to the right department, project, or cost centre in the accounting system.
Approval information: Who authorised the PO and when. In a well-governed AP process, this is recorded in the system rather than as a signature on a paper document.
The quality of the PO determines the quality of the matching process downstream. A PO with vague line item descriptions, missing quantities, or incorrect pricing creates exceptions at the matching stage that require manual investigation. Getting the PO right at the point of creation is the most cost-effective point at which to prevent AP errors.
An invoice is a document created by the supplier. It is a formal request for payment for goods delivered or services completed. Where the PO is the buyer's commitment to pay, the invoice is the supplier's claim on that commitment.
The invoice arrives after the goods or services have been provided. Its job is to confirm what was delivered, at what price, and to request payment within the agreed terms.
A correctly formatted invoice should reference the PO number from the buyer so that the two documents can be matched automatically. When a supplier issues an invoice without a PO reference, matching it to the correct purchase order becomes a manual task, which is one of the most common sources of delay in AP processing.
A complete invoice typically includes:
Nearly 40% of invoices processed manually contain errors. Many of those errors trace back to a mismatch between what the supplier has written on their invoice and what the buyer agreed to in the purchase order. When the two documents are not compared systematically and automatically, those errors progress toward payment.
The PO and the invoice are not isolated documents. They are the start and end of a workflow that connects the decision to buy with the act of paying. Understanding that workflow is essential for any AP team that wants to process invoices efficiently.
Step 1. Purchase request and PO creationA member of the business identifies a need and submits a purchase request. Once approved, a PO is generated and sent to the supplier. The PO is now the reference point for everything that follows.
Step 2. Delivery and receipt confirmationThe supplier delivers the goods or completes the service. The receiving party confirms receipt, which creates the delivery note: the document that confirms what was actually received, as distinct from what was ordered.
This three-document structure, PO, delivery note, and invoice, is the foundation of the 3-way matching process. Each document confirms a different thing: what was committed to, what was received, and what payment is being requested.
Step 3. Invoice receipt and processingThe supplier submits their invoice, referencing the original PO number. The invoice enters the AP system, either through manual data entry or, in an automated environment, through intelligent data extraction that reads the document without templates.
Step 4. Matching and approvalThe invoice is compared against the PO and the delivery note. Price, quantity, and supplier identity must align within configured tolerance thresholds. Matched invoices proceed to the approval workflow. Exceptions are flagged for review with the specific discrepancy identified.
Step 5. Payment and reconciliationThe approved invoice is scheduled for payment. Once paid, the transaction is reconciled against the ledger and the full audit trail, from PO creation to payment confirmation, is complete.
Three-way matching is the verification process that sits at the heart of the AP workflow. It confirms that the invoice is legitimate, accurate, and corresponds to a real transaction that has already been approved and fulfilled.
The three documents involved are:
When all three align, the invoice is approved for payment. When they do not, an exception is created. The most common discrepancies are:
Price mismatches: The invoice price differs from the PO price, either because the supplier has updated their pricing or because of an error. The correct resolution depends on whether the new price was agreed prior to invoicing.
Quantity discrepancies: The invoiced quantity differs from the delivery note quantity. This often occurs with partial deliveries, where the supplier invoices for the full order but only part has been received.
Missing delivery note: The invoice arrives before the delivery is confirmed. The correct approach is to hold the invoice until receipt is recorded, not to approve it based on the PO alone.
In a manual AP process, this matching is done by a team member comparing three separate documents, often across different systems. The average manual invoice processing cycle takes 14.6 days. Automated 3-way matching reduces this to under 3 days in best-in-class operations, and catches discrepancies consistently rather than depending on individual attention under time pressure.
Dost's AP automation platform handles 3-way matching automatically across purchase orders, delivery notes, and invoices, with configurable tolerance thresholds and real-time exception routing that surfaces discrepancies with full context for the AP team to resolve.
Purchase OrderInvoiceWho creates itThe buyerThe supplierWhen it is createdBefore goods or services are deliveredAfter goods or services are deliveredPurposeTo commit to a purchase and authorise spendTo request payment for a completed transactionDirectionBuyer sends to supplierSupplier sends to buyerLegal statusOffer to purchaseRequest for payment (becomes a legal obligation when accepted)ContainsItem description, quantity, agreed price, delivery detailsItems delivered, quantities, amounts due, VAT, payment termsRole in APReference document for matchingThe document being processed for payment
The key relationship between the two: the invoice should correspond to the purchase order. If it does, payment can proceed with confidence. If it does not, the discrepancy needs to be understood before payment is released.
Yes, and frequently is. Non-PO invoices are common for ad-hoc purchases, low-value transactions, and services where a formal procurement process was not followed. However, non-PO invoices carry more risk in AP because there is no pre-approved commitment to match against. They require additional verification, including confirmation from the relevant budget holder that the purchase was authorised, and they cannot go through an automated 3-way matching workflow. Businesses with high proportions of non-PO invoices typically have higher exception rates and more AP overhead than those with disciplined PO processes.
It becomes a matching exception and should be held until the discrepancy is resolved. The resolution depends on the cause: if the supplier has changed their price without prior agreement, the invoice should be queried and a corrected version requested. If the business has agreed to a price change that was not reflected in the original PO, the PO should be updated before the invoice is approved. If the discrepancy is within the configured tolerance threshold, the invoice can proceed automatically. What should never happen is approving a mismatched invoice without understanding why the amounts differ, because that is the gap through which duplicate payments and invoice fraud most commonly pass.
A purchase order becomes legally binding when the supplier accepts it, either explicitly through an order confirmation or implicitly by beginning to fulfil the order. At that point, it is a contract: the buyer is committed to paying the agreed price for the agreed goods or services, and the supplier is committed to delivering them on the agreed terms. This is why the PO is so important in accounts payable: it is not just an internal document. It is the commercial commitment that the invoice must correspond to, and that the business is legally obligated to honour once the supplier has performed.
A purchase order and an invoice are different documents serving different purposes at different points in the same commercial cycle. The PO is the buyer's commitment before delivery. The invoice is the supplier's claim after it.
Understanding the distinction matters practically because the connection between the two is where AP accuracy either holds or breaks. When every invoice can be matched against an approved PO and a confirmed delivery note, the AP process is controlled, auditable, and efficient. When invoices arrive without a corresponding PO, or when the two do not match, manual investigation absorbs the time and attention that automated AP should be eliminating.
The 3-way matching process that connects these three documents is one of the highest-value automation points in the AP cycle. Getting it right, systematically and at scale, is what separates finance teams that control their payables from those that are controlled by them.
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