68 Hemi Gtx

I just got a tip today on a 68 HEMI GTX. I don't know yet if it's a 4 spd or auto. The guy wants a $500 finders fee before he gives me the contact info. He is certain that the owner will sell. I was thinking of drawing up a contract with him, something like if I can't seal the deal he gets no money. What is customary for this type of thing?