Lady Shay wanted this guy to do some work for her but she was not willing to pay him. She felt that he had made enough from other people and he had to do it for her for free. When he refused, she told him that she would fart on him and wet farts to be specific. He got scared and he acted accordingly as he knew she was not messing around.
Lady Shay was angry at her slave and she did not want him to continue messing with her. She chose to fart and make him smell it. That was both a punishment and also a way to express her anger. The alternative was to beat him up and he did not want that. So he smelled the farts and knew he did not have any room for further mistakes so he changed.
Lady Shay loves fart fetish but she is not the type to dominate someone without good cause. And since this guy had not done anything to her that would have warranted her to fart on him, she chose to pay him to make him smell her farts. And since he needed the money, the guy agreed and so the mistress got to enjoy her fart fetish and the guy got the money he needed.