Is it permissible for the fifth brother, who relinquished his share of the land, to return and demand a share from his siblings after many years have passed since his relinquishment, during which his siblings disposed of their shares by building upon them and selling them?

It is known among jurists that whoever waives a right of theirs and forfeits it, being an adult and of sound mind, is not entitled to reclaim it, because what has fallen does not return. If a person forfeits a right that they are permitted to forfeit, that right is forfeited and does not return after its forfeiture. The Sharia judiciary is the one that settles such disputed cases.