How is Zakat on money calculated for a business partnership whose profits have not yet been received, for loaned money that has not yet been repaid, and for savings from a monthly salary whose amount constantly changes, and when does the hawl (one lunar year) calculation begin for them? And is Zakat calculated for past years in which it is difficult to determine the amount of money?

The money you trade with is subject to Zakat at the end of every Hijri year, by estimating your share of the goods and their profit and giving out Zakat if it reaches the nisab (minimum threshold) by itself or by combining it with your cash. As for the money loaned to your brother, Zakat is due on it for the past years once you receive it. Regarding savings from your salary, it is easier to pay Zakat when a Hijri year has passed on the first amount that reached the nisab; this is an permissible advance payment. However, if you desire precision, then pay Zakat for each installment of savings when its own year has passed. You must calculate the Zakat for past years in which the money reached the nisab and a year passed on it, and then pay what is due from you. If you are unable to ascertain with certainty, then act based on diligent investigation and strong presumption. Zakat is not obligatory unless a year has passed on an amount that reached the nisab. If the amount falls below the nisab during the year, the year (for Zakat calculation) is interrupted, and if it reaches the nisab again, the year begins anew from that point.