Personally, I don't like random sex but it has always bothered me how having a sex with a random girl is perfectly fine while doing the same thing and thanking her for her efforts afterwards with money would make it a criminal act (at least it's the case where I live).
If she is a slave girl, though, I can see the reason. It's her intended function and reason to exist to make her body useful to anyone her owner approves. And unless she was either reserved by her owner, she should have spread her legs to any free person without even thinking, if she was ordered to do so.
So, pretending she was a free person and asking for money for doing what she should do for absolutely nothing constitutes a crime and she needs to be punished by that.
An important distinction here - a whore is a free person, because she is able to charge for her body which means she owns it. A slave, on the other hand, cannot own anything, not even her breasts or cunt as they justly belong to her owner. She is a property herself so she cannot own even her own body, not to mention of any moeny.
If she acts like she's forgetting this simple fact, she needs to be reminded of it.