Is zakat obligatory on saved money, or on what a year has passed on, after deducting expenses from it?

Money that has reached the nisab (definition) and a full Hijri year has passed on it, its zakat (definition) must be promptly paid and not delayed except for a valid excuse. If the owner spends this money after the year has passed, its zakat is not forfeited; rather, it remains obligatory upon him because it is a debt upon him. The zakat must be paid on all money that a year has passed on, and spending it afterward does not nullify the zakat. If the paid rent was a debt before the year passed, it can be deducted from the money, and then the remaining amount is subject to zakat. However, this is a point of contention among scholars, and the more cautious approach is not to deduct debt from zakat at all.