A friendly tone does not turn an unfair request into a reasonable one.

Warmth is easy to confuse with fairness. A request delivered kindly, with apologies and appreciation attached, can still be asking for something you cannot give, and the niceness makes refusing feel like an attack on the person rather than an answer to the question. That is often not manipulation, just habit. But the effect is the same: you find yourself agreeing to the tone.

Separate the two before you answer. Write down what is actually being asked, in plain words, without the wrapping. You can be as warm in your no as they were in their asking, and the warmth costs you nothing once the answer is clear.