I say, with Allah's guidance: If the debt is expected to be repaid due to the honesty of the debtor or the existence of proof, such as a debt acknowledged by a wealthy person, or an insolvent person, or a bankrupt person, or a denier with evidence against him, or known by a judge, then zakat for the past years is obligatory when the creditor receives his money. However, if the debt is not expected to be repaid due to the debtor's denial or lack of proof, such as lost money, money fallen into the sea, money seized without evidence, money buried in a forgotten place, a debt denied by the debtor for years and then acknowledged later, or confiscated money received after years, then zakat is not obligatory. This is because for zakat to be obligatory, the wealth must be fully owned both in essence and in disposal, which is not the case here. However, if the money is returned after years, zakat is due only for the year it is returned, as explained in Sharh al-Wiqayah, p. 208. And Allah knows best.