Is it permissible to pay half of the mahr (dowry) in advance and half at a later date in the marriage contract, or must it all be paid in full at the time of marriage?

It is permissible for the dowry to be prompt or deferred, or a portion of it prompt and a portion deferred, according to the agreement of the spouses, and there is no harm in that. The deferred dowry becomes a debt owed by the husband, to be paid upon the arrival of the agreed-upon term or upon separation (divorce, annulment, or death). The basic principle is to name a portion of the dowry at the time of the contract.