This is actually a tricky question. Your assault & battery law probably says something like "unconsented or violent touching," so the fact that you consented do

This is actually a tricky question. Your assault & battery law probably says something like "unconsented or violent touching," so the fact that you consented doesn't matter, so you've been assaulted. And you can't enter a contract to perform an illegal act, so you don't owe $300 -- it's not a good contract.[1] But it depends why you're being slapped! Obviously we can't go prosecuting everyone who intentionally whacks into someone else according to the rules of whatever sport they're playing, so we make an exception for that. This gets tricky with sports like boxing where the rules require you to do something that looks pretty much like a brutal assault, so the exceptions in that case get pretty finicky -- we might insist that you make your mutual-battery deals in public, in licensed boxing rings, instead of by giving Brad Pitt a nice manly handshake in the basement of a bar and refusing to talk about it later. And then there's the whole gray area of less socially-accepted sports, like Quidditch or BDSM. We (even prosecutors) often approach things from the perspective of, "if this activity seems normal to me, it must be legal; if it seems wrong it must be illegal." When moral judgments and fear of the unknown creep into that analysis, it can lead to some twisted logic to justify the outcome we know must be right. [1] Now consider whether you can be prosecuted for A&B as an accomplice.