If you owe delayed Zakaah and the value of the currency has dropped you don’t give more. As for taking the drop- in currency into consideration and making up the difference, this applies to debts owed to people, because people would not tolerate these differences and would dispute the matter. Debt should be paid off in the same amount as was borrowed, not on the basis of its value, even if the value of the currency drops, so long as the currency is still valid and is not obsolete.