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