What is the Islamic legal ruling concerning a wealthy defaulter regarding an old debt in a currency whose value has significantly depreciated, and is he obligated to repay in foreign currency at today's exchange rate?

The debtor must repay what was established as a debt at the time it was incurred, regardless of its value at the time of repayment. The debtor's procrastination does not obligate him to repay the debt at its current value. The creditor has the right to demand repayment and refer the matter to a judge to compel the debtor to pay, as long as the debtor is able.