I own a flat that I have given to my parents-in-law to live in without paying rent, as they are unable to do so. The stipulated rent of the flat is Rs. 10,000 per month. Can I consider this Rs 10,000 as zakah paid to my parents-in-law?

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. 

It is invalid to consider the rent of the flat in which you accommodate your wife's parents, based on the assumption that you did not let them live in it for free, as part of your zakah. The element of Tamleek, i.e. physical transfer of zakah to the hand of its recipient, is missing. The zakah must be paid to the poor so that they may freely dispose of it. The Hanbali scholar Al-Buhooti wrote, “For the zakah to be valid, the condition that the poor takes possession of it must be met. It is not sufficient to give the poor lunch or dinner from the zakah money because this is not possession.”

If you have rented the flat to your wife's parents and the rent has become a debt upon them and they are unable to repay the debt, then according to a large number of scholars, it is invalid to cancel the debt against the zakah. Dr. Wahbah Az-Zuhayli wrote:

“As for the scholars who held the second view, i.e. the majority of the scholars, they believe that writing off or canceling the debt of the insolvent debtor as a substitute for paying zakah is not allowable, nor is it sufficient in terms of clearing one's liability from the obligation of zakah. Rather, the person should pay the zakah to the rightful recipient. The same applies to the case in which he repays the debt of a dead poor person against the zakah; this is not considered zakah because the element of Tamleek is missing...” [Al-Fiqh Al-Islaami wa Adillatuh, 3/1986]

It seems that you have accommodated your wife's parents in your flat for free, as evidenced by your statement, “I own a flat that I have given to my parents-in-law to live in without paying rent, as they are unable to do so.” If this is the case, then the past rent is considered a gift from you to them, and they have taken the gift into their possession; hence, it is impermissible to revoke what they have already possessed of that gift.

As for considering this rent as voluntary charity, then that is valid. Any act of kindness falls under voluntary charity.

Allaah knows best.