Is it permissible for a father to stipulate the division of the remainder of his land among his children by mutual consent, after taking an undertaking from the two sisters, for whom houses had already been built, that they would not claim their share in the remaining land?

A gift to an heir is not valid unless there is a legitimate شرعي justification. In this case, the gift should be returned, or the remaining heirs should be given what achieves justice. Scholars have differed on the issue of equality between male and female (heirs in gifts). The preponderant opinion is that there should be equality between male and female in giving. If one does not act justly during their lifetime, justice becomes obligatory after their death, and the one who was favored must share justly with their siblings.