Is it permissible, or an innovation (bid'ah), to repeat the marriage contract (nikah) and stipulate a new dowry (mahr) that is less than the first dowry, before the wedding night? And is the husband obligated to give this new dowry to his wife?

There is no basis in Sharia for repeating the marriage contract (Nikah), and the second contract is invalid, adding nothing new; the bond between the spouses is established by the first contract. As for the Mahr (dowry), it may be given in advance, deferred, or divided into installments, according to the agreement between the spouses, or between the husband and the wife's guardian, or according to prevailing custom, based on the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions." The Mahr and its method of delivery should be explicitly agreed upon to avoid dispute, and the woman's guardian should lighten the Mahr.