All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The price of this apartment is considered a debt on you to the company with which you signed a contract to build it.
Scholars held different opinions regarding whether the debt should be deducted from the total sum liable to Zakah or not; please refer to Fatwa 87557 for the details on the scholarly opinions about this.
The most cautious opinion is that of the scholars who held that the debt should not be deducted from the total sum liable to Zakah; instead, the person should pay the Zakah on all his properties liable to Zakah regardless of the debts to which he is liable.
Allaah Knows best.