Q 2: Is it permissible
to pay Zakah retroactively?
I mean is it permissible for a man owning a Nisab (the minimum amount on which Zakah is due) who has delayed paying the due Zakah for years to pay the Zakah due on the money for the past years? How can such a person pay Zakah when he is not sure of the exact amount on which Zakah was due at that time?
A person who delays payment of due Zakah without a valid Shar`y (Islamic legal) excuse is sinful. In fact, there are many evidences from the Qur'an and Sunnah to the effect that a person should proceed to pay Zakah once it is due.
A person who delays the payment of Zakah once it is due has to pay it even if the delay took place for years. To do so, this person should assess discretionarily the amount of money of the Zakah due which was not paid for years. In other words, he should assess discretionarily both the amount of money as well as years, in case he doubts its number too. In this regard, Allah (Exalted be He) said:
So keep your duty to Allâh and fear Him as much as you can
(Part No. 9; Page No. 396) May Allah grant us success. May blessings and peace be upon our Prophet Muhammad, his family and Companions!