Rather than a direct no, some version of "Yes, if" usually works better. Yes, I can do that for you, if. If I'm given the resources to do that job, and I'm reli

Rather than a direct no, some version of "Yes, if" usually works better. Yes, I can do that for you, if. If I'm given the resources to do that job, and I'm relieved of some existing work. Yes, if some prerequisite can be delivered by other people. Just lay out honestly what it would take to drop existing work and fulfill this additional request.