Zakaah that has not been paid for several years

To find out the nisaab on
which zakaah must be paid, please see question no.
2794. 

With regard to paying
zakaah for past years, whoever is obliged to pay zakaah and delays it with
no excuse has committed a sin, because the evidence of the Qur’aan and
Sunnah indicates that zakaah must be paid immediately at the time when it
becomes due. So he has to repent sincerely to Allaah. 

Secondly, whoever is
obliged to pay zakaah and does not do so at the specified time has to pay it
later on, even if he has delayed it for a number of years. 

“If he is certain about
the amount then he should pay it, and if he is not certain about the amount
then he should pay an amount with the intention of paying zakaah, until he
thinks that what he has given is most likely to cover the amount of zakaah
that he owes. Basing actions on what is most likely to be correct is one of
the basic principles of sharee’ah.” 

(The Standing Committee.
See Fataawa Islamiyyah, vol. 2, p. 95) 

He should make an
estimate if he does not know the exact amount, and pay that. He also has to
repent. And Allaah is the Source of strength.