Is an employed sister entitled to claim full ownership of an apartment in exchange for a sum of money less than half its value, after the mother reneged on her promise to deed the entire apartment to her?

The apparent meaning of the question is that the mother favored the questioner's sister in the sale of the apartment. Favoritism is a type of gift (hibah), and in gifts, it is necessary to be just between children according to the most preponderant opinion. It is not permissible to favor some of them without a legitimate reason. She must rectify the situation either by giving the remaining children the same amount she gave to the sister, or by annulling the sale if the sister agrees to it. It is not lawful for the favored one to take the excess; rather, she must share it fairly with her siblings. Since the sister refused to annul the sale, the mother does not have the right to annul the contract because the sale was validly concluded with the consent of both parties. The reason for the mother's reconsideration is not an unfair price but rather her feeling of not being just among her children. This does not affect the validity or binding nature of the sale. The mother must give the remaining children the same amount she gave to the sister. If she is unable to do so, we ask Allah Almighty to forgive her.