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.