Zakaah on the later portion of the mahr
Question

                Is there any zakaah on the later portion of the mahr?.
Praise be to Allah.The later portion of the 
    dowry is a debt that the husband owes to the wife. The view about zakaah on 
    it is like the view on zakaah on all other kinds of debt. One of two 
    scenarios must apply with regard to this dowry: 
-1- 
If the wife is able to ask 
    for it and her husband is rich and has no objection to giving her the later 
    portion of the dowry, then zakaah should be paid on it every year, because 
    it has become like the wealth that she possesses. 
-2- 
If the husband is poor and 
    cannot pay it, or the wife cannot ask for it for fear of making her husband 
    angry and he will divorce her if she asks for it, or if the custom is that 
    the woman is not entitled to this delayed portion unless she and her husband 
    become separated either through divorce or death – as is customary among 
    people nowadays – then no zakaah is due on this delayed portion because she 
    is not able to dispose of it. But when she does get possession of it, she 
    must calculate one year from the time when she took possession of it. In 
    order to be on the safe side, she should pay zakaah for one year when she 
    takes possession of it, even if many years have passed. 
Shaykh al-Islam Ibn 
    Taymiyah (may Allaah have mercy on him) was asked about the dowry of a woman 
    who had been married for many years but she was not able to ask her husband 
    for it lest they become separated, then she was compensated for her dowry 
    with some real estate or by being given the dowry after many years. Does she 
    have to pay zakaah for the past years or when one year has passed since she 
    took possession of the dowry? 
He replied: Praise be to 
    Allaah, there are many opinions concerning this matter among the scholars. … 
    The most correct opinion is the view of those who say that no zakaah is due 
    at all until one year has passed, or zakaah is due once when she takes 
    possession of it. There is evidence for both views. The former is the view 
    of Abu Haneefah and the latter is the view of Maalik, and both are mentioned 
    in the madhhab of Ahmad. And Allaah knows best. End quote from Majmoo’ 
    al-Fataawa (25/47). 
And Allaah knows best.