Princess Isabella wanted to scare this guy and she did it because he had messed up. She knew that not doing anything about it was akin to encouraging him to mess up and she did not want to be party to such a thing. That is why she chose to use her findom fetish to degrade him. As she did it and burned a hole in his pocket, he learned his lesson.
Princess Isabella needed to tease this guy and turn him into a paypig in order for her to get some financial benefits out of him. The mistress knew and she was sure that the guy would fall for her and he did. He was then turned into a human credit card without him noticing. That is how the guy paid her rent without him even noticing how it all happened.