Mahr, is an amount of possessions which a man is bound to give his wife for marrying her.
Allah the Almighty commands:
“Give the women [upon marriage] their gifts of companionship (mahr) graciously. But if they give up willingly to you anything of it, then take it with pleasure.” (An-Nisa/ The Women 4:4)
Mahr can be determined by mutual agreement. It doesn’t have any maximum or minimum. It can be a house, a car, a ring or some amount of money. It can consist of anything. The one thing that is necessary in determining it, is the mutual consent.
If the spouses did not agree on a Mahr during marriage, it counts as a debt and it is determined by looking to other marriages and the amounts other brides get. While doing this, the couple should consider people that live close to their standards.
Mahr is determined between husband and wife. No other family member can interfere in it. They can only give advice but the last decision is to be made by the bride.
Lastly, it is strongly advised to write down the amount, payment method, etc. for the Mahr; as Allah advises to write down debts. (2:282)