No zakaah is due on money if you are not certain that you will get it.
Question

                We are employees in a petroleum company in Saudi. We were sent abroad for three years. Five years after we returned we knew that we have financial rights for the years we were abroad. We complained to the individuals in charge, doubting that we will get these rights. One year later we were given these amounts. Do we have to pay Zakat on them? If yes then shall it be for six years (since we returned) or for one year only which is the year we complained during? As we received the money in the end of this sixth year.
Praise be to Allah.
    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.