Dowry-

The dowry, known by the Arabic terms mahr or sadaq, is one of the rights which a wife is owed by her husband. The fuqaha' define it as the money which a woman is owed by a man on account of marriage or sexual intercourse.

As for it being money which is owed to the woman on account of marriage, that is self-evident. However, the fact that it is owed by the man for sexual intercourse needs furthur explanation. That occurs when a man has sexual relations with a woman based on the erroneous supposition that she is his wife. It is what the Fuqaha' call "erroneous intercourse" and would happen, for instance, if a woman is given to a man on the basis that she is his wife and then after consummation it becomes clear that she is not. In such a case, the man owes the woman the dowry of a woman of her standing. This is also the case when a man has sexual relations with a woman afetr making an invalid contract with her, inasmuch as the contract does not fulfil the fundamentals and preconditions necessary for the marriage contract. In this case the dowry of a woman of her standing must also be paid by the man.