How can the value of Zakat be calculated for a commercial shop for past years in the absence of accurate records of income and expenses, and is it permissible to pay it in installments or from money other than that generated by the shop, and is there a specific time required for its payment?

Praise be to Allah who has bestowed upon you repentance, for withholding zakat is one of the greatest sins, and delaying it after it becomes due without a valid excuse is among the forbidden acts.

You are obligated to calculate the zakat that was due from you in past years and to pay all of it, because zakat is not waived by the passage of time; it is a debt upon your conscience.

If you are unable to ascertain the exact amount with certainty, then strive to investigate and review your accounts, then pay what will bring certainty or strong probability of clearing your conscience.

The general rule is that it is not permissible to pay zakat in installments, and one must hasten to pay it unless there is an excuse, such as the unavailability of funds, in which case its payment is delayed until the funds become available.

There is no specific time for paying zakat, and one must hasten to pay it when capable. It is permissible to pay zakat on trade goods from the principal of the store, without including the profit.