How is the overdue Zakat, due from the Zakat payer for 20 years, calculated? Is the calculation based on the amount that first reached the nisab and remained in his possession, or on the value of the money each year? And what should he do if he is uncertain about the amount in one of those years?

Zakat must be paid immediately. It is not permissible to delay it except for a valid excuse. If the zakat is a debt, then the beginning of the year for the first quorum (nisab) should be considered. So, if a person owned money and did not pay its zakat for several years, then the money perished or fell below the nisab, he must pay the zakat for each year separately based on the amount that was present in that year. Even if the money decreased in one of the following years or perished before the completion of the year, none of the zakat from the previous years is waived. If there is doubt about the exact amount of money for which zakat is due, then zakat must be paid for the amount that is certain.