What is the ruling on paying Zakat for past years based on a single Hawl (Zakat year) despite the fluctuating Nisab (minimum amount on which Zakat is due) during those years, and is it obligatory to recalculate the Zakat for the correct Hawl, and what is the Hawl considered for future years?

If the wealth falls below the nisab during the hawl (zakat year), the hawl is interrupted. If the nisab is met again, a new hawl begins from the date it was met again. If an error is discovered in the calculation of zakat for past years, and there was a deficit in what was obligatory to be paid, then it must be paid now to one of the eight categories of zakat recipients. It is not obligatory to pay it to the same previous individuals, and there is no sin upon the one who made an error in the calculation and did not intentionally delay it.