Princess Serena was experimenting with different body parts and money fetish. She wanted to see which of her body parts were going to help her make more money from her money fetish. That is because she wanted to be exciting and unpredictable as opposed to using the same style and technique over and over again to do her thing. And that is how and that is why she chose to try her ass.
Princess Serena was interested in dominating this loser and she knew the best way to do it was financially. So she went ahead to tease the guy and to get him to do that which she felt he needed to do. The mistress forced the poor guy to pay her for services rendered and she did not let him determine the cost. She did that by herself and emptied his account.
Princess Serena shook her money maker and she used it to tease as well as to humiliate this loser. The guy did not know what she did to him but he was turned on and he could not resist or refuse anything she told him. He was at her beck and call and he did all she asked, including bankrolling her. The naughty mistress loved how it all turned out.