What is a promissory note and should I use one for an unpaid invoice?
Promissory notes for unpaid invoices — when they help and how to use one
Short answer
A promissory note is a signed written promise to pay a specific amount by a specific date or on a payment schedule. Converting an unpaid invoice into a promissory note does three useful things: it creates an unambiguous acknowledgment that the debt exists (which resets the statute of limitations clock in most states), it specifies exact repayment terms (removing ambiguity about when payment is due), and it gives you a cleaner legal instrument to sue on if the client defaults — courts treat promissory notes as near-conclusive evidence of the debt. Use one when a client cannot pay immediately but commits to a payment plan, or when an invoice is old enough that you want to reset the statute of limitations.
Invoices establish what was owed and when. But an invoice, by itself, does not document the debtor's agreement that the amount is correct. A client can still dispute an invoice even after receiving it. A promissory note is signed by the debtor — by signing, they confirm the principal amount, the interest rate (if any), and the repayment schedule. That signature closes off most factual defenses: the debtor acknowledged the amount, agreed to pay it, and accepted the terms.
Statute of limitations reset is one of the most valuable functions of a promissory note. If an invoice is 2–3 years old and you are approaching the limitations window, a signed promissory note (which is itself a new written contract) typically resets the clock from the note date. You are not extending credit again — you are memorializing an existing obligation in a new instrument. This preserves your right to sue for additional years. Consult the specific rule in your state, as some states treat acknowledged debt differently from newly executed promissory notes.
Interest specification matters. An invoice may or may not include a late-fee or interest clause from your underlying agreement. A promissory note can explicitly specify the interest rate on the outstanding balance, making the accruing interest contractually documented rather than contested. For a client repaying over 6–12 months, making the interest rate explicit (e.g., 8% per annum) turns a slow payment situation into one that at least compensates you for the delay.
What to include in a promissory note for an unpaid invoice: the principal amount (the invoice balance), the interest rate (zero if you choose), the repayment schedule (lump sum or installments with specific dates), the original invoice numbers and dates the note is intended to satisfy, the debtor's full legal name and entity type (individual or LLC), and signatures of both parties with dates. The note should be dated the day it is signed, not backdated. You do not need an attorney for a simple promissory note below small-claims thresholds, but for amounts above $25,000, having an attorney review it takes 30 minutes and costs far less than a collection failure.
When a promissory note is NOT the right tool: if the client disputes the underlying invoice, do not ask them to sign a note for the disputed amount before resolving the dispute — courts may not enforce a note signed under duress or without genuine agreement on the underlying debt. Resolve the dispute first, then document the agreed amount in a note if needed. Also, do not use promissory notes as a substitute for timely follow-up on current invoices; they are a restructuring tool for late situations, not a routine billing practice.