Some person used to save his money in a box without paying its due Zakah

Q: My father died a month ago and left about (Part No. 8; Page No. 350) 180,000 riyals. Our mother informed us that for five years our father had saved this sum of money in a box without paying its due Zakah (obligatory charity). We roughly estimated the period over which this money was accumulated by the time of our younger brother's marriage, which took place in 1416 A.H. My father had spent all his savings on that marriage and since then he started to save his future earnings in a box. My question is:First: How should we calculate the due Zakah of these five years without knowing the real figure of his savings in each year? Second: How can our father's failed obligation to the payment of Zakah prior to these five years be discharged? He did not use to pay Zakah. Rather, he used to consider the unpaid debts by people as Zakah.I hope Your Eminence could answer these two questions.

A: If it is certain that your father did not pay his due Zakah of the above mentioned sum of money, it becomes obligatory upon you to calculate his due Zakah for all the past years and pay it immediately in order to discharge him of his failed obligation. You should pay 2.5% of money which remains after paying Zakah of each previous year. Zakah is a debt to be paid to Allah. Neither his nor your obligation can be discharged except by paying Zakah to its deserving beneficiaries.May Allah grant us success. May Peace and Blessings of Allah be upon our Prophet Muhammad, his family and his Companions!