A quiet no can preserve affection that a resentful yes would slowly damage.

Agreeing against your own judgment postpones the cost and increases it. The favor gets done grudgingly, the grudge leaks out in tone and timing, and the other person receives help alongside a faint sense of having asked for too much, which is worse than being told no. A refusal is a single clean event. Resentment is an ongoing one.

Say no this week to the thing you would have agreed to and regretted. Keep it short and warm, with no case for the defense, and notice how much easier the friendship feels afterwards.