Sheikh Muhammad Muhammad Sâlim `Abd al-Wadûd
If the money had already been in the purchaser’s possession for a year before he made the purchase of the house, then Zakâh is due upon it without a doubt. Even if he had paid the money to the seller of the house at the time of the sale, the back-due Zakâh will still have to be paid.
If Zakâh was not yet due upon the purchaser’s wealth at the time he concluded and finalized the purchase, then there are two situations:
1. He has physically separated the money for the purchase from his wealth and specified it as the sole property of the seller. He can do this by placing the money in a separate account or by putting it in a separate envelope.
In this case, he will not owe Zakâh on this wealth. The seller will owe Zakâh upon it after it remains in the seller’s possession for one year after receipt.
2. The price of the house remains mixed with the purchaser’s other wealth. In this case, he will still be liable to pay the Zakâh upon it.
And Allah knows best.
Source: Islam Today