What is the ruling on a sister selling her inherited share to her brother for a price that later turned out to be much less than the prevailing market price at the time of sale? Is the contract valid, and how can the seller reclaim her right after the contract has been registered with the court?

If the sales contract between a sister and her brother for all that she inherited from her father's estate occurred before the division of the inheritance and the determination of the daughter's share, this is called "Mukhārajah" (relinquishment/quitclaim), and some scholars permit it. A condition for the validity of Mukhārajah is that the inheritance generally be known; because selling an unknown (thing) is impermissible. However, some madhhabs (schools of thought) permit reconciliation over an unknown thing in specific cases or absolutely, such as the Hanafis, who do not stipulate that the actual items of the inheritance be known if they do not require possession. If the daughter is mature and has mutually agreed with her brother, then there is no objection to that, and in case of dispute, recourse is made to the Sharia courts.