What is the legal (Shariah) amount for a deferred dowry (mu'akhkhar al-sadaq), and is zakat obligatory upon it if it reaches the nisab?

The amount of the dowry has no bearing on the obligation of zakat upon it. There is no maximum limit for the dowry, but it is preferable to reduce it in accordance with the Sunnah and to facilitate marriage. The dowry may be less than twenty mithqals, and yet zakat becomes obligatory on it if it reaches the nisab by being combined with other wealth owned by the woman.

As for the zakat on the dowry before it is received, it is a debt owed by the husband. If the husband is solvent and willing to pay, the woman pays zakat on it every year. Otherwise, she pays zakat on it when she receives it. According to the majority of scholars, a debt owed by a solvent person is subject to zakat along with one's other wealth. However, if the debtor is insolvent, zakat is paid upon its receipt for the past years, according to the majority. Abu Hanifa, on the other hand, holds that a new year (for zakat calculation) begins upon receipt, while Malik believes that zakat is paid for only one year upon its receipt.