Is Zakat obligatory on a salary from three years ago, which was received in a single payment and reaches the nisab?

If the salary is due monthly but was not received because the recipient was outside the country, then zakat for past years must be calculated as per Fatwa no. 98644. However, if the salary was not due, and was given as a donation after the return, then zakat is not obligatory on it until a full year has passed from the date of its possession. At that point, a quarter of a tenth (2.5%) is to be paid as zakat.