Mistress Medea needed to find a way to dominate the guy and she did it in a way that she had never noticed it being done. The mistress wanted to make sure that things worked out the way she wanted. That is why she went out of her way to make sure that the guy learned his lesson and that he drank her pee. The mistress recorded everything he did.
Mistress Medea had an unruly slave who needed to be tamed. And since she was busy and she did not have a lot of time to dedicate to him, she chose to pee on him and make him change. He had to drink her urine before she let him go. And the slave had no choice but to change as he knew she would stop at nothing to ensure he changed.