There are times when you can't say no even though you don't have time to do what you're asked for.
|Sometimes you're pressed to say or do something, ask for time to consider the request rather than rushing into troubles|
Buy time and say you need to consider the request. Be clear on when you'll get back with a time estimate.
This will give you some time to bring what you're working on to a pause then do an plan of your future availability.
The good thing is that it shows you're taking the request seriously, not trying to change subject.