Is it permissible to use gold valuation or inflation indexing to determine the value of an old debt, taking into account that 100 ten years ago is equivalent to 200 today, and would this be considered usury (riba)?

The debtor is obligated to repay what became due from him at the time the debt was incurred, regardless of the currency's value at the time of payment, whether it has risen or fallen. It is not permissible, Islamically, to agree at the time of contract signing to link future debts to a cost-of-living index, or to gold and silver, or to another currency, or to interest rates; because that leads to gharar (excessive uncertainty), jahalah (ignorance), lack of equivalence, injustice, and disputes, unless the currency completely ceases to exist, in which case the value of the debt at the time of its cessation becomes obligatory.