Is the brother entitled to claim the difference in the price for the increased area of the land that he relinquished to his brother, especially since the original contract and the power of attorney mention a smaller area? And does this increase rightfully belong to the relinquishing brother or to the brother who paid for the land and received it?

If a land is sold based on a specified area, and then it turns out to be larger, the increase belongs to the seller. The buyer is only entitled to what was agreed upon in the contract. This increase is the property of the seller, and purchasing it requires setting a price that satisfies the seller, for Allah, the Almighty, says: (O you who have believed, do not consume one another’s properties wrongfully but only [in lawful] business by mutual consent) and for the saying of the Prophet, peace be upon him: (Indeed, selling is by mutual consent).