Is it permissible to annul a sale due to the seller's severe disadvantage (ghabn) and his ignorance of the land's value at the time of sale, based on the hadith "Defrauding the trusting is usury (riba)"?

If your uncle sold his share of the land to your father, and he was of sound mind and no one deceived him, then he has no right to revoke the sale on the pretext that he was unaware of the land's value at the time of sale. This is the view of the majority of scholars. As for the Malikis, they have two opinions regarding revoking a sale due to excessive deception (ghabn fahish), with detailed conditions. The hadith "Deceiving a trusting person is usury" (Ghabn al-mustarsil riba) is weak. Even if it were authentic, the 'mustarsil' (trusting person) is one who trusts the seller and asks him to sell something for what it is truly worth, so deceiving him would, for some scholars, establish the right to revoke the sale. Therefore, your uncle has no right to revoke the sale unless your father agrees to annul it (iqalah), which is religiously recommended.