Is my signature on a contract for the transfer of apartment ownership in the name of the heirs—after the death of my brother who sold it to me—considered a relinquishment of my right to the apartment, especially since my intention was to register it temporarily in the name of the heirs to prove ownership in the cooperatives, with the commitment to pay the amounts they expended?

Issues of disputes and conflicts are presented to Sharia (definition) courts or verbally to scholars so that they may hear from both parties and ascertain the arguments. As for a mere fatwa (definition), it may not suffice to resolve the dispute, and it is advised to take the case to court. Since the apartment belongs to the questioner based on a fatwa from Dar al-Ifta, they are more knowledgeable about what they say. Registering it in another person's name does not change the reality of the matter, and a ruler's judgment does not alter the truth of the reality. For whoever is judged to possess something that is not rightfully his brother's has indeed taken a piece of hellfire.