Make room for the other version
Ask what the other person understood before assuming that you are arguing about the same facts. For a message about how to decline a request to lend money, use this factual base: acknowledge what was requested, any real timing issue, and whether your answer applies only now or more broadly.
Correct the record precisely
Correct only what you can support, acknowledge any real impact, and propose a practical way to prevent repetition. Give a direct no, a limited alternative only if you mean it, and any practical information they still need. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Agree on a testable next step
Do not overexplain private details, blame another person, or say maybe when the answer is no. When pressure, coercion, or retaliation is possible, use a safer channel and involve appropriate support. Do not recruit an audience, publish private evidence, or turn a misunderstanding into a campaign about character.