First of all, if you’re talking to a manager, they will love you if you stop saying “nah I can’t do that” and start saying “I have X and Y on my plate right now and I think I could do Z for you if you and my manager think I should drop Y for it. What do you think has higher priority?”
By phrasing your “no” this way, you’re setting up the conversation for a collaborative tone, you’re recognizing that everything can be traded off, and you’re helping the requester understand what levers they have to work with.
Say no with context