Is a youth considered sinful if he believes he has fulfilled his debt but his assumption is incorrect?

If a debt is established, it remains a liability until it is paid or discharged. If the debtor doubts whether he has paid the debt and the creditor does not acknowledge payment, the debt remains the debtor's liability. As for the son's statement that he remembers his father paid the debt, while the father himself does not remember, this is a lie for which the son incurs sin. He must seek Allah's forgiveness and repent to Him.