Complain about Michigan agent?

[Sharad M.]

My cousin just bought a place in Michigan and the agent he was using promised him 50% of his commission for using him. My cousin bought a place for $260k and his agent got a commission of $7,800. After the close the agent refused to pay him and said that he got a great deal on the house and he shouldn't have to pay anything. Now he has agreed to pay $400 when he owes $3,900.

Any experience on how to handle this situation?