بسم الله الرحمن الرحيم
(Fatwa: 1194/1185/N=11/1437)
(1) The minimum quantity of mahr is ten dirham i.e. 30 gram 618 milligram silver or anything of same value. Mahr is not lawful less that it. If the mahr was fixed less than it then nikah shall be valid but it shall be necessary to give the least amount of mahr.
(أقله عشرة دراهم) لحديث البيهقي وغيره: لا مهر أقل من عشرة دراهم، ………، (فضة وزن سبعة) مثاقيل كما فى الزكاة، ……. (وتجب) العشرة (إن سماها أو دونها) (الدر المختار مع ردالمحتار ، كتاب النكاح ، باب المهر ، 4: 230-233 ، ط: مكتبة زكريا ديوبند).
(2) If the husband, Qazi and witnesses did the nikah on a mahr of less than ten dirham unknowingly and nikah was solemnized with witnesses then it is their ignorance. They must get necessary knowledge of nikah. They should have learnt the necessary rules of nikah from some reliable Mufti.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband