How is a down payment from 20 years ago to be returned, given the change in the Algerian Dinar's value? Should it be returned at its nominal value (nine million) or at its purchasing power at that time?

The principle is to repay debts with their equivalents at the time of repayment, and the depreciation or appreciation of their value is not considered, as long as they are still in circulation. Money and what is considered equivalent to it, with which transactions are conducted, are guaranteed by their like. So, whoever had money from a sale or a loan, and it was abolished, only its equivalent is to be sought. The resolution of the Islamic Fiqh Academy stipulated that the standard for repaying fixed debts in a certain currency is by its like, not by its value, and it is not permissible to link debts to price levels. Therefore, as long as the currency in which the debt was determined is still in existence and being transacted with, the amount should be returned to its owner as it is.