Ali bin Abd al-Aziz (may Allah have mercy on him) said that Abu Ubayd (may Allah have mercy on him) related the hadith of Sayyiduna Ali (may Allah be pleased with him) concerning a man who is owed a doubtful debt. He said: He pays zakah on it for the past (years) when he receives it, if he is truthful.
Al-zanun means a debt about which its owner does not know whether the debtor will repay it or not, as though he has no hope of it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7623
Abdullah bin Dinar narrates from Ibn Umar (may Allah be pleased with them both) that he said: Pay zakah on the wealth that is in your possession; and a debt that is with a reliable person is like what is in your possession; and wealth that is in the form of a doubtful debt has no zakah on it until it is received.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7624
Humayd bin Abd al-Rahman (may Allah have mercy on him) said that Abd al-Rahman bin Abd al-Qari (may Allah have mercy on him) was in charge of the treasury of Sayyiduna Umar (may Allah be pleased with him). He said: People used to take zakah from debts, in this way: when the stipends were issued, the overseers ('urafa') held back people's debts for them, and on whatever remained in their hands the zakah was taken out before they received it. Then, after that, people extended loans that went bad, so they did not take zakah from a debt except what was realised in cash; but when they received the debt, they paid its zakah for the time that had passed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7625
Ayyub bin Abi Tamimah al-Sakhtiyani (may Allah have mercy on him) said that Umar bin Abd al-Aziz (may Allah have mercy on him) wrote, concerning wealth that some governors had seized, that it should be returned to its owners and its zakah taken for the years that had passed. Then, after that, he sent another letter saying that only a single zakah should be taken from it, because it was dimar; and this means wealth whose recovery is not hoped for.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7626