How is the amount of an endowment in a sold house estimated, and is it permissible for the heirs to take their share of the agreed-upon endowment amount?

Dealing with endowment (Waqf) issues is complex and requires recourse to the Sharia courts. The answer to your question depends on the status of the room:

1. If the room was taken possession of by the beneficiary during the lifetime of the endower, then the endowment is valid, and it is not permissible to sell it except for a benefit determined by the supervisor of the endowment or the judge.

2. If the beneficiary did not take possession of it, and it remained in the possession of the endower until his death, then there is a difference of opinion among scholars regarding the condition of the beneficiary taking possession before the death of the endower. Sheikh Al-Islam mentioned two opinions:

One: The endowment is void, and this is the madhhab of Malik and Ahmad in one of the two narrations, and Abu Hanifa and Muhammad.

Second: The endowment becomes binding, and this is the madhhab of Al-Shafi'i and Ahmad in one of the two narrations, and Abu Hanifa and Abu Yusuf.

Therefore, a fatwa is not sufficient in this matter, and recourse must be made to the Sharia judiciary.