Mehr is a "gift" that a husband promises his wife at the time of marriage. It is specified in the aghd which is a pre-nuptual agreement. A wife is entitled to request her husband give her mehr at any time after they get married, and he must give it to her at that time. If a man divorces his wife, she is also entitled to her mehr.
Unlike dowry, mehr is not given to the brides parents, but rather to the bride herself, and remains her property for ever. Anything the bride brings into the household and buys during the marriage also belongs to her alone. A husbands property belongs to the family.