What is incumbent upon a university student who has a monthly allowance and a salary from her father, both of which fluctuate throughout the year with decreases and increases, but she does not know the exact amount upon which a year has passed for the last three years? And is there any unpaid Zakat due from her?

Zakat becomes obligatory on money if a lunar year has passed and it has reached the nisab (the equivalent of 85 grams of gold or 595 grams of silver). The amount to be given is 2.5%.

If you own a nisab of money and a lunar year has passed without it decreasing below the nisab, Zakat becomes obligatory on it, and it must be paid for each year separately. If you are not certain of the amount, then act based on your best judgment (ghalabat al-dhann).

Whoever is obligated to pay Zakat and delays it without a legitimate excuse commits a sin. They must pay it for all delayed years. If they doubt the amount of money or the number of years, they should act based on their best judgment, in accordance with the verse: "So fear Allah as much as you are able."

However, if you doubt whether a lunar year has passed on the money, or whether it reached the nisab, or if the nisab decreased during the year, then Zakat is not obligatory on you, because the original principle is the absolution of liability.