What is the jurisprudential ruling on how to calculate and pay Zakat for past years, taking into account the difference in place of residence (Saudi Arabia and Egypt) and the fluctuations in gold and currency prices between the two countries, and is it permissible to consider the Nisab based on the price of 21-karat gold?

The questioner should verify the accuracy of their understanding regarding their father's zakat situation and consult with scholars to clarify the matter. If they cannot precisely determine the nisab (minimum taxable amount) and the exact amount of wealth, they should exert their best effort (ijtihad) and act based on what they most likely believe, because whoever owes zakat and does not pay it on time must pay it for all delayed years. The general rule for paying zakat is that it should be in the same currency as the wealth. However, it is permissible to pay it in another currency, provided it is equivalent to its real value at the time of payment. A change in the purchasing power of the currency after it became due does not affect it. As for 21-karat gold, it is not pure. To determine the pure gold content, multiply the number of grams by 21 and divide the result by 24.