The payment of zakah is not obligatory on the one who is owed a debt by a person before it is in his possession, because it is not in his hands. But if the debt is owed to a wealthy person, then that person is obliged to pay the zakah of it every year. If he paid it along with the zakah on his wealth, then he has fulfilled his obligation, but if he did not pay it along with the zakah on his wealth, once he receives it , he is obliged to pay zakah on it for all the previous years. This is because the rich man could request its payment, but he left to the choice of the debtor. However, if the debt is owed to a poor person, or a rich man who is unable to demand its payment, then the one owed is not obliged to pay zakah upon it every year. This is because it is not possible for him to acquire it , and Allaah, the Most High says:
"And if the debtor is in a hard time (has no money), then grant him time till it is easy for him to repay." Al-Baqarah 2:280
So it is not possible for him to receive this money, and benefit from it, therefore there is no zakah upon it. However, if he received it there are those amongst the scholars who say he should begin a new hawl, while others say he should pay zakah for one year, and if a year passes, he should pay zakah for that too, and this is more prudent. And Allah Knows better.