What is the ruling regarding the right (to a share) if a brother waives his share in an apartment to his sister in exchange for the sale of another apartment, and is his signature on the contract binding on him, especially if he believes he was wronged?

If the sister agrees with her brother that the expert made an error in the inheritance valuation, then the inheritance should be re-divided in a way that achieves justice and mutual consent between them.

However, if the sister does not agree with her brother, then the matter remains subject to the judge's ruling. In this case, the brother will have the Tanta apartment, and she and her mother will have the Alexandria apartment, because: 1. The ruler's judgment resolves disputes: The judge obligates the litigants to abide by the issued ruling, even if one of the litigants believes they are in the right. As Sheikh Ibn 'Uthaymeen said, "The ruler's judgment removes disagreement and resolves disputes." 2. Matters of ijtihad (independent reasoning): Valuing real estate is a matter of ijtihad where discrepancies may occur. A judge's ruling in such matters cannot be overturned as long as it conforms to the principles of Sharia or allows for ijtihad, because it occurred in a place of ijtihad and was implemented. 3. Upholding judgments: A litigant does not have the right to accept a judgment and then dispute it later, because that would negate the benefit of the judiciary. Necessity dictates the binding nature of a judgment based on ijtihad, so as not to lead to continuous and unending disputes.

Therefore, there is no need to re-divide the inheritance if you do not agree that the expert erred. The matter remains as ruled by the judge. It is advisable to observe kinship ties and not let worldly matters become a cause for dispute.