If one fails to pay zakah, he is obligated to pay it for the years that have passed
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The second question of fatwa number ( 4792 )
Q2: Is it permissible to pay zakah retroactively? I mean, if a person possesses the nisab and does not pay the zakah at its proper time, and this is delayed for several years, is it permissible to pay the zakah for that elapsed time? And how can a person pay the zakah if he is not certain of the amount of wealth for which zakah was due at that previous time?
A2: A - One who is obligated to pay zakah and delays it without a prescribed excuse has committed a sin; because the evidence from the Book and the Sunnah commands hastening to pay zakah at its proper time. B - One who is obligated to pay zakah and does not pay it at its designated time is obligated to pay it later, even if his delay lasts for years. He pays the zakah on the wealth that was not zakked for all the years he delayed paying it, and he acts according to his estimate regarding the amount of wealth and the number of years if he is uncertain about them, based on the saying of Allah the Exalted:
So fear Allah as much as you are able
فَاتَّقُوا اللَّهَ مَا اسْتَطَعْتُمْAnd success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his companions.
Source
www.alifta.gov.sa
The Arabic text is copied verbatim from the original source, without any edits.