And Imam Hasan al-Basri (may Allah have mercy on him) said that if a person unknowingly marries a woman who is forbidden to him (mahram), then separation will be enforced between them, and whatever dowry she has already received will belong to her. She will not receive anything else except this, and after that, she will be given her mahr equivalent (mahr mithl).

Narrated Abu Mas`ud: The Prophet prohibited taking the price of a dog, the earnings of a soothsayer and the money earned by prostitution.