Is it permissible for the Mahr to be a debt (deferred) if the husband cannot pay part of it immediately?

Paper currency takes the place of gold and silver. Purchasing cash with other cash requires equality and immediate possession (qabd) if the type is the same, or immediate possession if the types differ. This is based on the Prophet’s (peace be upon him) saying: "Gold for gold, and silver for silver... like for like, equal for equal, hand to hand. If these types differ, then sell as you wish, provided it is hand to hand." Therefore, it is not permissible to buy gold with a deferred payment; rather, the full price must be taken possession of (qabd) at the time of the contract.