Should the claimant reclaim his right from his brother at the price at which the brother sold it, or at the price at which the claimant sold his land?

If your brother sold a part of your land without your permission, his action is an aggression and an unlawful usurpation. His sale falls under the category of the sale by a usurper, which is null and void, because he sold something he does not own.

If you approve the sale, then the price for which that part was sold is yours. If you do not approve the sale, then you have the right to reclaim the usurped part from the buyer, and the buyer should seek reimbursement from your brother for the price he paid. We advise you to be content with what your brother pays to preserve the relationship between you.