All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
According to the most preponderant opinion of the scholars Zakat on a loan should be paid, whether it is immediate or delayed to an appointed time, on the condition that the debtor is sure to repay. However, the scholars differed in opinion whether he should pay the Zakat on one lunar year or until he receives the amount and then he would pay the Zakat on all the previous years. In order to be on the safe side, it is better to opt for the first opinion. However, if he had to deal with a procrastinating person or someone who denies that he is indebted to you, then in this case a person is not obliged to pay the Zakat on it until he receives the money and then he should pay the Zakat for one year as explained in Fataawa 82524 and 84378.
However, there are some scholars who are of the view that he should pay Zakat for all the previous years, and it is better to act on this opinion in order to be on the safe side.
Allaah Knows best.