Does the charity I gave with the intention of paying off a debt—which I was unable to deliver to its owner years ago—suffice for the obligation of paying off the debt now that I have been able to reach him?

You must repay the debt to its owner, because his right does not diminish due to what you did, and your liability is not discharged except by repayment. This is unless the creditor is pleased with what you did and approves of your charity on his behalf with this money. If he is not pleased, the reward for this charity will revert to you, and you are obligated to pay him his money.