When a supplier’s invoice exceeds the purchase order: recovering an overpayment in Canada

If a supplier charged more than the agreed purchase order amount and you paid for work only partly completed, you can usually recover the excess money—but how you proceed depends on your contract, the evidence you have (especially a documented payment timeline), and provincial rules on court limits and time limits. Start by checking the purchase order terms and assembling invoices, payment records and communication showing partial performance; then use a formal demand or, if that fails, a small claims suit or a restitution claim such as unjust enrichment. Where a government contract is involved, administrative recovery rules may give the Crown faster tools to recover improper payments.

This article explains the practical steps that matter in Canada, the evidence that strengthens your case, the usual legal routes (contract claims and restitution), and the provincial caveats—using Ontario as an illustration for small claims and limitation-period rules. It also explains when a demand letter makes sense and what to expect if you proceed to court.

First, use the contract (purchase order) — it often decides the result

Your purchase order or written contract is the primary document. Look for clauses that set price, scope, invoicing rules, acceptance tests, holdbacks, and expressly allow repayment or set‑offs for incorrect invoices. If your contract contains an explicit repayment or reconciliation mechanism, that language will usually govern how you recover an overpayment.

When the contract is clear, a simple, pointed demand that cites the contract and attaches the purchase order, the supplier’s invoice, and your proof of payment often leads to a quick resolution. If the supplier resists, the contract language will guide a court or tribunal on whether the payment was authorised or must be returned.

(For federal procurement contracts, note that public procurement rules expressly reserve the Crown’s right to recover payments discovered to be incorrect after payment; see federal guidance on supply arrangements.) Government of Canada guidance on supply arrangements

Gather the evidence that courts and suppliers respect

A documented payment timeline is one of the most persuasive pieces of evidence. Build a concise bundle that includes:

• The original purchase order and any written amendments that change price or scope.
• Supplier invoices (with invoice numbers and dates) and any progress or interim bills.
• Proof of payments: bank records, cleared cheques, payment reference numbers and dates.
• Records showing partial performance: delivery notes, site reports, timesheets, inspection reports or an internal acceptance log that shows what was completed and what was not.
• Email or letter exchanges where the supplier acknowledges a dispute, admits partial completion, or requests payment terms.

A clear chronology—dates, amounts, invoice references and who approved each payment—lets you show the overpayment precisely and prevents disputes about timing or whether the payer accepted the work.

Try a formal demand before suing: why it matters and what to include

A firm, documented demand is usually the cheapest first step. Its purpose is practical: create a clear, written record of your claim, give the supplier an opportunity to correct billing or repay, and set a deadline. A good demand letter should:

• State the purchase order number, invoice numbers and exact overpaid amount.
• Attach or summarise the payment timeline showing when the supplier billed and when you paid.
• Quote the contractual clause (if any) permitting repayment or set‑off, or explain why the invoice is not authorised by the PO.
• Offer a reasonable deadline for repayment and state that litigation or a small claims filing is next if there is no response.

A demand letter does not guarantee payment, but it strengthens later court filings and is often enough to prompt negotiation, especially where the supplier wants to preserve the relationship or avoid a judgment.

(For examples of administrative recovery powers where the Crown is the payor, see federal procurement guidance.) Government of Canada guidance on supply arrangements

If a demand fails: small claims, contract claims, or unjust enrichment

Two legal routes are the most common:

• Contract claim: If the purchase order controls billing and the supplier breached those terms, sue for breach or a reconciliation of invoices to recover the difference. The contract often decides whether a repayment is owed.

• Restitutionary remedy (unjust enrichment / quantum meruit): If there is no enforceable contract for the disputed portion or the payment lacks any juristic reason, Canadian courts can order the supplier to disgorge the benefit. The Supreme Court’s modern statement of these principles is in Kerr v. Baranow, which explains the elements courts seek when ordering restitution. Kerr v. Baranow, Supreme Court of Canada decision

For modest sums, provincial small claims courts are the practical forum. In Ontario, for example, Small Claims Court handles claims up to $50,000 and provides forms and online filing guidance—useful when you simply want a judgment for repayment. Ontario Small Claims Court guide

Time limits and provincial differences — act promptly

Limitation periods vary by province and can bar claims if you delay. In Ontario the Limitations Act, 2002 generally gives two years from the date you discover the claim. That discovery rule matters: document when you learned of the overbilling and preserve a file date. Other provinces have different timeframes, so check the statute for your jurisdiction. Limitations Act, 2002 (Ontario)

If you wait beyond the applicable limitation period, even a strong factual case can be dismissed as time‑barred.

Practical choices when the supplier disputes your facts or is insolvent

If the supplier contests your records, the dispute turns on which evidence a judge finds credible: detailed payment timelines, contemporaneous emails admitting partial completion, and signed delivery or acceptance notes are strong. If the supplier is insolvent, a judgment may be difficult to enforce and you should seek legal advice about insolvency remedies and creditor priorities.

A realistic example

Imagine you issued PO #123 for $20,000 for installation work. The supplier invoiced $25,000 and you paid $25,000 in two payments. Site records show only 60% of work completed; inspection reports note outstanding items. Your file includes the PO, the $25,000 invoice, bank transfers with dates, and emails where the supplier promised to finish the balance. A clear demand letter attaching those documents and asking for $5,000 repayment or a plan is the natural first step. If the supplier refuses, you can pursue a contract reconciliation or a restitution claim in small claims court, provided you are within the provincial limitation period.

When to get a lawyer and final practical tips

Get legal advice if the sum is large, the contract is complex or contains ambiguous reconciliation clauses, the supplier raises counterclaims (for example, that you accepted the work), or the supplier is insolvent. Otherwise, a documented demand and small claims suit are common, cost‑effective routes.

Before you act: assemble the purchase order, invoices, payment records and a simple payment timeline; send a clear demand; and if that fails, file in small claims or seek restitution. These steps make the case factually and legally credible.

For professional help drafting a clear demand letter that organises your evidence and timeline, consider a specialised service that prepares dispute documents based on your information.

For help preparing a professional demand letter, BeforeSuit can help you prepare your next steps.