What should the debtor do with the money of his debt if he cannot find the creditor, and should he return the amount as it is or its value after the currency has depreciated?

If the currency's value has not changed, you must repay the lender the same amount. If you are unable to find him after making an effort, then give the amount in charity on his behalf. If you are able to reach him later, you must repay him, unless he waives his right to claim it.