Am I entitled to claim the remainder of my share in the tannery, which was sold to my brother at a price significantly lower than the selling price of the other shares, after I learned that he knew its true high value?

If a person sells something while ignorant of its value, and does not haggle over the price out of trust in the buyer, and then it becomes clear that he was defrauded (ghabn), he has the option of rescinding the sale due to ghabn. He is entitled to demand the difference in price or to annul the sale and reclaim the item. Ghabn is considered established if the price difference is significant, such as if it was sold for double its real price. This option applies to both the seller and the buyer if either of them was unaware of the market price. The defrauded party has the right either to annul the sale or to uphold it while receiving compensation for the ghabn.