DY, if the supply is the girl who signed the contract and is in fact prepared to pay the bucks, then the shadchan should not be hung up with the boy who does not necessarily have to sign a contract. She is chasing down the boys. They are in demand. So your little scenario is a false pretense and totally inaccurate. She is the buyer in this case and looking for the merchandise. She need not be afraid that she will get “stiffed” because she knows that the girl will be paying a hefty sum. If the boy stiffs her, then there is really nothing she can do about it. On the other hand, if she was a professional shadchan who ran a real business, both boys and girls would be coming to her, and the first thing she should do would be look into her own pool of candidates and try to make a match within. Then she would get paid from both clients.
Parents who believe in shadchanim would go that route as long as they are dealing with responsible and professional people who do the legwork and don’t just drop names.