If you were offered $100 to punch someone (no person in particular). So you get one of your friends and you say, "Let me punch you and I'll give you $50." If he says, "yes," is it a sin?
I'm puzzled about where this one is going. You see, I used to do Tae Kwon Do, and I've gotten hit lots of times. Hmm, come to think of it, I had to pay a fee for that "privilege," too. I guess I didn't realize you could make money off of being hit. I suppose stuntmen are being paid to take hits in movies.
In each case that comes to mind, there is a purpose behind the punching, whether it is training or the creation of entertainment. And in each case, it is controlled so that no real damage is done -- at least not intentionally. I can't figure out when a person would just offer money for one person to hit another.
But let's generalize it a bit. What if you were asked to harm another person. Would it be wrong if that other person agreed to it? Here the answer is clear. "Love does no harm to a neighbor; therefore love is the fulfillment of the law" (Romans 13:10). In Christianity, we are to treat other people as we want to be treated. Most people don't like getting hurt, so we ought not to hurt another person.