Set them free

Don't keep people waiting. If the answer is no then tell them no. Saying we'll see or let me see what I can do only gives them false hope that they have a 50-50 chance at you helping them with what they need, when in fact you know they have no chance at all. A honest no is better than 100 fake maybes. They could move on if you send them on their way. Even if the answer is no it will set them free.