Praise be to Allah.
If you agreed with your brother that he will pay you one million, for example – which is the price of the goods – in instalments over several years, then this amount is a debt that your brother owes you, so you should pay zakaah on the debt. There are well-known details concerning that, which are as follows:
1. If the debt is owed by someone who is well-off – meaning that he is able to pay off the debt and does not deny it – then you must pay zakaah on the debt every year, just as if the money was in your possession.
It is permissible to delay paying zakaah in that case until you receive the money, then when you receive it you should pay zakaah on it for the past years.
2. If the debt is owed by someone who is taking too long to pay, or he denies the debt, or he is poor and in financial difficulty and is unable to pay it back, then you do not have to pay zakaah on it until you receive it, then when you receive it you should start to count one Hijri year from the day you received it [and pay zakaah after one year]. If you give zakaah for one year when you receive it, that is better and more prudent.
As for the goods which were yours, it is obligatory for your partner to pay zakaah on them, because he bought them from you and they were transferred to his ownership on the day when he agreed with you that he would pay you for them.
See: al-Mughni (2/345); al-Mawsoo‘ah al-Fiqhiyyah (23/238).
And Allah knows best.