What is the ruling on zakat for money paid to own an apartment in installments after a year has passed?

Zakat is obligatory for whoever owns zakatable wealth that reaches the nisab (minimum threshold) and a hawl (full lunar year) has passed over it, unless they have a debt that prevents the wealth from reaching the nisab after its repayment.

So, if you have already bought the apartment, and you do not have enough money to reach the nisab after deducting the value of the apartment that you owe, then no Zakat is due on you, whether the installments are due immediately or deferred. Debt prevents Zakat if it depletes the wealth or leaves less than the nisab.

However, if you have not yet bought the apartment, but rather saved money to buy it, then Zakat is obligatory on you, provided there is no other impediment.