The wish compels her to do as he says.

Megan tried to resist but her body wouldn't let her. She dropped to her knees in front of the old guy. She grabbed his cock in one hand and opened her mouth. She licked the tip, then slowly put her lips around the head. She carried on like this until the old man grew impatient and **** her head down deeper. He moved his hip forward as he pushed her head down back and forward. Before she could pull herself away, she had the full length of his dick down her throat, and he held her there for what seemed to Megan like minutes. She began to ****. He then pulled her off by the hair and stroked his dick to completion, coating Megan's face in cum as he moaned.
Megan was sat there in stunned silence as the man got up, pulled up his pants and threw her some money out of his pocket. "Keep the change, whore." He said as he moved to leave her room.
"I wish I wasn't a prostitute!" Megan was finally able to say. The man disappeared, along with his cum. Unfortunately so did the money.
Megan stayed on her knees for a while, attempting to process what happened. "I really can make wishes come true." She thought, "But I need to be really careful what I wish for." She got up and looked at herself in the mirror again. She thought about the slutty dress and stockings. Those aren't the kind of clothes she wanted. She also thought about the money that had disappeared. Could she wish for more money?