It's fraud, plain and simple, imo. If there was an understanding that she'll get paid afterwards, and he doesn't pay, then that's fraud.
I dont know the inns and outs of prostitution, but if it's common to pay after the deed, then no, it wasn't rape. The prostitute would be willingly have sex, with the expectation of being paid afterwards.
If the mutual understanding is to pay up front though, and he forces the woman into having sex while ensuring her she'll get paid afterwards, then that's another story though.
And theft is the taking of another person's property without their consent. He didn't take anything from her per se, though. He didn't give her what was rightfully hers, yes, but that's slightly different, I think. If he payed her and took the money back from her, I believe that would then count as theft though.
@OP: I cant begin to wrap my head around someone paying 500 bucks to get kicked in the crotch, when he can jump on a hand rail instead to save it (or, uuh, not hurt his crotch to begin with
).
That said; why would a prostitute sue for sexual harassment for being asked to kick someone in the balls? She could just refuse it with the money (ah right, murica
). If she took the money on the agreement and didn't fulfill it, she ought to be sued for fraud though.