Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to ask someone for a debt repayment plan, use this factual base: list the amount, date, reason, receipt, prior agreement, and any part that remains genuinely disputed.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Propose a payment, refund, evidence exchange, or installment plan with dates both sides can assess. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not threaten public exposure, add invented fees, or treat hardship as proof of dishonesty. Use secure payment methods and qualified banking, consumer, legal, or fraud support when appropriate. You do not need the other person to agree with a boundary for you to follow it consistently.