Ruling on the Zakah for the previous years wherein one did not pay it out of laziness

Q: A civil servant depends on his monthly salary to meet the needs of his living expenses as well as other necessary expenses, such as the costs of his marriage, preparing, furnishing, and paying the rent of an apartment for residence and so on. He deposits what remains of his salary in one banking company and by the end of the year, he pays the due Zakah (obligatory charity) on the total sum he saved. He had been doing this (Part No. 8; Page No. 337) for a long period of time: about 12 years. He is a religiously committed person and observes the Five Obligatory Daily Prayers and believes that Zakah is a Rukn (integral pillar) of Islam but he did not pay the Zakah due on him for some years. It is not out of his denial of its obligation but out of laziness. He sometimes pays only a portion of the due Zakah and keeps the rest until he has forgotten the number of years during which he did not pay the due Zakah and the sums of money that he used to own during these years.After his Tawbah (repentance to Allah) for having delayed the payment of Zakah, he regrets the years when he was lazy and did not pay it. Does his Tawbah exempt him from paying the Zakah for the past years on the grounds that Tawbah deletes the past sins? If not, how should he calculate the Zakah after he forgot the exact number of years and the sums of money that were with him during each of these past years? May Allah reward you with the best in this life and in the hereafter.

A: He should pay the Zakah due on the years that he did not pay totally or partially. If he does not know the amount of the due Zakah for these years or the sums of money, he should be careful to free himself from blame but he is not exempted from paying the Zakah just because its due time has passed because it is an obligation that Allah (Exalted be He) ordains to be given to specific legal recipients. Therefore, he is free from this obligation until the Zakah is paid. He should offer sincere Tawbah from his total or partial abandonment of paying Zakah due on his money and be resolute repeat this. He should also do more supererogatory acts of `Ibadah (worship) and pay (Part No. 8; Page No. 338) more Sadaqah (voluntary charity). May Allah accept his repentance and replace his sins with good deeds.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.