That is a pretty restrictive request, a descent of 1500'. Normally I'd ask an aircraft to stay below the level it's already at. But I have to ask, if you didn't want to do it, why did you agree to it? We aren't mind-readers, and we do appreciate that you are trying to achieve certain objectives from your flight too.
'Negative, unable. Request position of the traffic please, I'll see if I can get visual contact' perhaps? Or an offer to take up a different track?