Mistress Maryan wanted to grow and push her agenda and she did this with her scat fetish. She used her shit to dominate this guy and she had it in his bread. She used it to make him a sandwich and the poor guy had no choice but to do what the mistress wanted. It was humiliating but he learned his lesson and he did what she wanted him to do.
Mistress Maryan wanted to make sure that her client did not try any nonsense. He had tried to go back on his word and to mess her up and she was not going to allow that as they had allocated a lot of time and resources on the project for it to be abandoned midway. That is why she chose to make him eat her shit and to drink her pee.