What is the method for deducting apartment installments from savings to determine the amount on which Zakat is due, and am I excused for what I wrongly deducted in past years, and what is the ruling on deducting installments from savings on which a year has not yet passed?

Debt is not deducted from zakat according to the preponderant opinion, which is the madhhab of the Shafi'is. Those who held that debt is deducted from zakat, most of them stipulated that the person should not possess any other wealth exceeding his needs that could be used to cover the debt. Those who advocated for deducting debt differed among themselves: some said the entire debt is deducted, some said that deferred debt is not deducted, but only immediate debt is, and some said that only the due installment is deducted. Therefore, there is no blame on the questioner for what has passed. However, in the future, he is obliged to pay zakat on all his wealth without deducting debt, because zakat relates to wealth, and debt relates to personal liability, so one does not preclude the other.