How is the zakat calculated for past years, taking into account that the money exceeded the nisab despite withdrawals made during the year, and is it permissible to pay the due amount in installments, given that it is a debt?

To determine the zakat due for previous years, refer to the bank and request an account statement for the past years to ascertain the amount of zakatable wealth for each year. Whatever is now possible to pay from the zakat that was obligatory in the past must not be delayed or paid in installments. If the money runs out and you are unable to pay it now, it becomes obligatory to pay it whenever it becomes feasible. Allah Almighty says: "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life, and We will surely give them their reward [in the Hereafter] according to the best of what they used to do."