What is the ruling on zakat for a sum of money that constantly changes due to lending parts of it to friends and acquaintances, some of whom are non-Muslims, with some of them delaying repayment for more than six months? And what about the amounts that I lent for the sake of Allah, which amount to a quarter of the total sum?

Zakat (definition) becomes obligatory on wealth if it reaches the nisab (definition) (threshold) and a full Hijri year has passed over it. The nisab is equivalent to 85 grams of pure gold. The amount of Zakat that must be paid is one-quarter of one-tenth (2.5%).

As for the debts owed to you by others, Zakat on them is obligatory upon you because they are still your property. If the debt is owed by a solvent and willing person, you pay Zakat on it along with the money you possess. If the debt is owed by an insolvent, procrastinating, or denying person, you pay Zakat on it when you receive it for the past years, or when you receive it for one year, or you begin a new year (for calculating Zakat) with it, according to the differing opinions of scholars.