Does one sin by delaying the repayment of a debt for which no due date was agreed upon, which led to the departure of its Christian owner, who was considering Islam, and how can this situation be rectified?

The answer to the question involves two issues: First, regarding the due date for repaying a debt if it was not agreed upon, it reverts to the customary practice of people. Second, concerning whether a debt must be paid upon ability, even if the creditor does not demand it, there is a difference of opinion. The preponderant view is that it is obligatory to pay immediately, out of fear that the repayment might be missed due to inability to reach the creditor. Therefore, it is necessary to search for the address of the creditor and promptly send the payment to her.