Invoice Basics

Invoice vs Purchase Order: Key Differences Every Freelancer Must Know

Most freelancers have heard of purchase orders but aren't sure how they differ from invoices β€” or why corporate clients reject invoices without a PO number. Here's the complete guide.

πŸ“‹ Purchase Order (PO)

  • Issued by: The buyer (your client)
  • Direction: Buyer β†’ Seller
  • Timing: BEFORE the work starts
  • Purpose: Authorizes the purchase
  • Legal effect: Binding commitment from the buyer
  • Content: PO number, approved vendor, items, approved amount

🧾 Invoice

  • Issued by: The seller (you, the freelancer)
  • Direction: Seller β†’ Buyer
  • Timing: AFTER the work is done
  • Purpose: Requests payment
  • Legal effect: Demand for payment for goods/services rendered
  • Content: Invoice number, services, amount, PO reference, bank details

What is a Purchase Order (PO)?

A Purchase Order is a document generated by the buyer's procurement or finance system that formally authorizes a purchase. It says: β€œWe, the buyer, approve spending up to X amount with Y vendor for Z services.”

Think of it as the buyer's internal approval document. Before anyone at the company can commit to spending money with an outside vendor, they need a PO approved internally. The PO number is the reference that ties everything together.

A PO typically contains:

β€’PO number (unique identifier)
β€’Date issued
β€’Buyer name + address
β€’Approved vendor (you) + address
β€’Description of services/goods
β€’Approved quantity and unit price
β€’Total approved amount
β€’Billing and delivery instructions
β€’Authorized signature
β€’Budget code / cost center reference

What is an Invoice?

An invoice is a document you (the seller) send to request payment for goods or services already delivered. It's a formal legal demand for payment.

For freelancers, an invoice is how you get paid. It documents what you did, what you charged, and gives the client the information they need to pay you (your bank details, due date, etc.).

An invoice contains:

β€’Invoice number (sequential)
β€’Invoice date
β€’Your name/company + address
β€’Client name + address
β€’PO number (if applicable β€” critical!)
β€’Description of services
β€’Quantity and rate
β€’Total amount (+ VAT if applicable)
β€’Payment terms (Net 30 etc.)
β€’Your bank/payment details

The Relationship Between PO and Invoice

In a formal procurement process, the PO and invoice are two sides of the same transaction:

1
PO Created
Client raises PO and sends you the PO number
2
Work Done
You complete the services
3
Invoice Sent
You invoice, referencing the PO number
4
3-Way Match
AP matches PO + invoice + delivery receipt
5
Payment
Client pays the invoice

⚠️ The 3-Way Match β€” Why Your Invoice Gets Rejected

Large companies use a β€œ3-way match” process: their AP system automatically matches your invoice against (1) the PO, (2) the delivery confirmation or timesheet, and (3) the invoice itself.

If any of these don't match β€” wrong PO number, different amount, different vendor name β€” the invoice is automatically put on hold. This is why being exact matters: invoice your full name/company name exactly as it appears on the PO.

When Clients Require POs

Not all clients use purchase orders. Here's who typically requires them:

Large corporations (500+ employees)β€” Have formal procurement departments
βœ… Almost always
Government / public sectorβ€” Legally required in most jurisdictions
βœ… Always
Mid-size companies (50–500 employees)β€” Depends on their finance setup
⚠️ Often
Startups and SMBsβ€” Usually informal β€” email approval is enough
❌ Rarely
Individual freelance clientsβ€” No procurement department
❌ Never
Agencies (hiring you as subcontractor)β€” Especially if they have enterprise clients
⚠️ Sometimes

πŸ’‘ Best practice:For any new corporate or government client, ask upfront: β€œDo you need to raise a Purchase Order before I begin work?” Save yourself weeks of payment delays.

What If There's No PO?

Many freelance clients don't use purchase orders at all. This is completely normal for small businesses, startups, and individual clients.

For clients without POs:

  • Get written approval via email before starting work (this is your protection)
  • Agree on scope, price, and payment terms in writing (even informal email is fine)
  • Send a proposal or quote for them to approve β€” that written approval = your authorization

When corporate PO is delayed:

  • Don't start work until the PO arrives (if possible β€” protects you legally)
  • If you must start, get explicit written authorization from a named decision-maker
  • When the PO arrives, reference it on your invoice even if you've already done the work
  • Follow up on the PO status weekly β€” β€œPO is still being processed” is often a delay tactic

Freelancers and POs: Practical Tips

Always ask for the PO number before the engagement begins

At the start of every corporate engagement: β€œI'll need your PO number to reference on my invoice β€” can you confirm when that will be ready?” This flags to the client that you know the process and they can't use PO delays as an excuse later.

Reference the PO number exactly as written

Copy the PO number character-for-character. Even a missing hyphen (β€œPO-2026-0472” vs β€œPO20260472”) can cause the 3-way match to fail and your invoice to be put on hold.

Match the invoiced amount exactly to the PO

If the PO authorizes Β£5,000 and you invoice Β£5,100 (because of additional work), the AP system will flag a mismatch. Either get an amended PO, or split your invoice into two: one for Β£5,000 (referencing the original PO) and one for the Β£100 excess (referencing a change order).

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Frequently Asked Questions

What is the main difference between a purchase order and an invoice?

A purchase order is created by the buyer BEFORE work starts, authorizing the purchase. An invoice is created by the seller AFTER work is done, requesting payment. The PO is the buyer's commitment; the invoice is the seller's demand for payment.

Can I send an invoice without a purchase order?

Yes, absolutely. Most freelance and small business clients don't use POs. A PO is only required when the client's procurement process demands one. For clients without a PO system, a simple email approval is sufficient authorization to proceed.

Why do large companies reject invoices without a PO number?

Large companies use 3-way matching in their accounts payable systems: the invoice must match an approved PO and a delivery receipt. Without a PO number, the AP system can't match your invoice to an authorized purchase, so it gets put on hold automatically.

What happens if a client issues a PO for less than what I end up invoicing?

You'll need an amended PO (change order) approved before invoicing for the higher amount. The AP system will flag a mismatch if your invoice exceeds the authorized PO amount. Always communicate scope changes and get written approval before doing the additional work.

Is a purchase order legally binding?

Yes. When a seller acknowledges a PO, it becomes a legally binding contract between buyer and seller. The terms of the PO (price, scope, delivery) become the contract terms. If the buyer then refuses to pay, you can reference the PO as proof of the agreed transaction.

Do freelancers need to issue purchase orders?

No. As a freelancer, you're the seller β€” you issue invoices, not purchase orders. Purchase orders are issued by your clients. If you subcontract work to other freelancers, however, you'd be the buyer and might issue POs to them for your own record-keeping.

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