Praise be to Allah.
So long as you do not have possession of the second part of the dowry, you do not have to pay zakaah on it. When you take possession of wealth that reaches the nisaab (minimum threshold) and one year passes since it came into your possession, then you have to pay zakaah on it even if you are saving it or keeping it to furnish the apartment and so on, unless you pay it towards furnishing the apartment or you give it to your wife as her dowry before one year has passed, in which case you do not have to pay zakaah on it.
The delayed part of the dowry is regarded as a debt that is owed to the wife by the husband, so the ruling on its zakaah is like the ruling on any other debt that a person owes to someone else. This is subject to further discussion among the fuqaha’:
If the debt is owed by someone who is able and willing to pay it off, and he can afford to pay it back when asked to do so, and he is not denying it or trying to delay it, i.e., you can get your money back whenever you ask for it, then zakaah must be paid on it every year, because it comes under the same ruling as wealth that is in the possession of the owner.
If the debt is owed by a poor person who has no wealth, or one who is denying it, and you have no proof of it, then no zakaah is due on it according to the more correct opinion, but when the owner takes possession of it, then in order to be on the safe side he should pay zakaah on one year, even if many years have passed. This has been discussed in the answer to question no. 1346.
Based on this, the delayed portion of the dowry (with which the furniture is to be bought) is a debt that you owe to your wife, so no zakaah is due from her or from you because you do not own that wealth.
Similarly, no zakaah is due on the delayed portion which the wife will not take unless there is a separation, because she cannot ask for it so long as she remains your wife, but if she decides to pay one year’s zakaah on it in order to be on the safe side, that is better.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: Is it correct to delay giving the wife her dowry? Is it a debt owed by the man that he must pay? Is zakaah due on it?
He replied: It is permissible to delay the dowry and there is nothing wrong with that, because Allaah says (interpretation of the meaning): “O you who believe! Fulfil (your) obligations” [al-Maa'idah 5:1].
Fulfilling obligations includes fulfilling them and adhering to their conditions.
If a man stipulates that all or some of the dowry will be delayed, there is nothing wrong with that, but it becomes due if the contract stated that it is due on a specific date, or if no specific date is mentioned it becomes due when separation occurs as the result of divorce, annulment of the marriage or death of the husband. It is a debt owed by the husband that may be demanded when the specified time come if he is still alive, or after he dies, as with any other debt.
Zakaah must be paid by the woman on this delayed dowry if the husband is well off and is willing to pay the debt, but if he is poor then she does not have to pay zakaah.
End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (18/30).
And Allaah knows best.