If I went up to a kid and was like 'hey kid, can I buy your bike for $50' and he was like 'yeah sure that sounds great I agree' ... I'd be pretty fucking pissed off if he was going around telling everyone I ripped him off when I bought his bike and trying to pressure people into giving him an additional donation.

For fucks sake kid, just say no to the $50. Say yes when you actually agree to the pay. It's really not hard, and it's all completely and totally optional.