Praise be to Allaah.
If the situation is as you describe in your question, then you do not have to pay zakaah, because you are not certain that you will get this money. It is like a debt owed by one who is going through financial difficulties. The correct view is that no zakaah is due on it until its owner takes possession of it and one full year has passed. The same applies to the money that you mention; no zakaah is due on it until one year has passed since you took possession of it. End quote.
Majmoo’ Fataawa Ibn Baaz (14/39).
It says in Fataawa al-Lajnah al-Daa’imah li’l-Ifta’: If the debtor is in financial hardship or is well off but he is taking a long time to pay it back, and the debtor cannot get it back from him, either because he has no proof that would enable him to prove his rights before a judge, or because he has proof but there is no one in authority who can help him to get his rights back, as happens in some countries where there is no support for people’s rights, then the debtor does not have to pay zakaah until he has taken possession of what is owed to him and one hijri year has passed. But if the debtor is well off and it is possible to get the debt back from him, then the creditor must pay zakaah every time one year passes, if the debt by itself or in addition to other money etc reaches the nisaab. Fataawa al-Lajnah al-Daa’imah (9/191).
And Allaah knows best.