Is zakat due on the 16,000 Saudi Riyals that was a trust held by the son and used by him for ten years until the father's death, and are the father or the son sinful for not paying its zakat?

The debtor must take the initiative to repay the debt, and if unable, should intend to repay it when capable. Zakat (definition) is not obligatory for the creditor on the debt before receiving it. If the debt is owed by someone who is well-off, then upon receiving it, the creditor should pay zakat for all past years. However, if the debt is owed by someone in hardship or who is procrastinating, then upon receiving it, the creditor should pay zakat for one year only. There is no sin upon the creditor for delaying the payment of zakat on the debt until it is received. The heirs' waiver of the debt is permissible if they are legally competent adults, but if there is a minor among them, his right to the inheritance is not forfeited.