Is the waiver of the heirs to their siblings residing in the inherited building, as is—which makes some of them tenants under an old law and others under a new law—permissible in Sharia, and what is the ruling on this agreement and its legal name in Islamic Sharia?

The answer is divided into several paragraphs related to your question about the heirs residing in the building (inheritance) and their legal and Sharia status.

1. Gift from the Father: If the father gifted one of his children an apartment or other property during his lifetime, and the gifted child fully possessed it during the father's lifetime, then it is a valid gift if he gifted similar properties to his other children or if there was a reason to specify the gifted child. However, if these conditions are not met, the gift is void and is included in the estate. 2. Status of the Heirs: If the gift was valid, the property belongs to the gifted person. However, if the gift was void, the property is part of the estate and must be divided by mutual agreement, usufruct sharing (muhayat), or lottery. 3. Resorting to Judiciary: If all heirs agree on a legitimate matter and no one disputes it, there is no need to resort to a judge or lawyer. However, if what was agreed upon is illegitimate, then it is obligatory to strive to change it. 4. Waving Lawful Rights and Participating in Unlawful Acts: It is obligatory to change unlawful acts and not participate in them. If it is possible to rectify the situation, then it must be done. However, if the situation is based on an unjust law and cannot be changed, then it is not described as unlawful because it falls outside the scope of religious obligation (manat al-takleef). 5. Name of the Legitimate Agreement: Not every agreement has a specific legitimate name. Rather, agreements are described as legitimate or illegitimate according to their particulars.