Three turns and a check. Keep this open next to your chat: each step is the thing to paste next, with what you typed already carried across. Nothing is sent from here, and nothing on this page is a transcript. The conversation stays where you are having it.
The request in the client's own words, with any context they gave.
The real reason, including the part that is about you rather than about them.
The client, their sophistication, and how this relationship has gone.
Anything relevant about the relationship or previous asks.
Whether this is email, a letter, or a call, and when they need an answer.
Watch for, the whole way through
A refusal that rests on an ethics rule or a conflict is not a negotiation and should not be written as one. Confirm the rule yourself rather than relying on any description of it here.
The model will soften a no into something that reads as a maybe. Read the first sentence and ask whether a client in a hurry would understand that the answer is no.
Doing it once as a courtesy sets the scope, whatever the engagement letter says. Decide deliberately rather than by default.
This message may be read later by someone deciding whether you handled the matter well. Write it so that reader would think you did.