Answer
(Fatwa: 963/716=D/1430)
Nikah takes place if adult boy and girl solemnize the Nikah in the presence of two Muslim witnesses, though it will be against sunnah not to publicise and not to recite the khutbah. Likewise, if mahr (dower) was not mentioned then also the Nikah was valid, but the husband has to at least 31.600 gm silver. If the girl gives up her demand of mahr before nikah, it will not be acceptable.
(2) Mahr can be agreed upon after Nikah as well, but it should not be less than the amount mentioned above.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband