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 apologize for making an unauthorized purchase, use this factual base: state the amount, agreement, transaction, and current shortfall without hiding behind vague language.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a written repayment, correction, or reporting plan with realistic dates and records. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not ask for trust before the numbers are clear or make a promise that depends on uncertain income. Use qualified financial, legal, banking, or fraud support when ownership or authorization is disputed. You do not need the other person to agree with a boundary for you to follow it consistently.