Does the last owner of the product deserve 300 Riyals—the amount he paid—or 500 Riyals—the full refunded amount? And does the original buyer deserve anything?

The second buyer has the right to return a defective product to the first seller, even if he cannot return it to its original source. This is because the rule is that every buyer returns to the one who sold him the item. If the second buyer became aware of the defect after the ownership of the product transferred from him to another party, he is not entitled to financial compensation, according to the more sound opinion, because the product may be returned to him, in which case he can return it. If ownership returns to him, he has the right to return the product, whether it returned to him by way of return due to the defect or by any other means. The second buyer does not have the right to return the product directly to the first seller because he did not purchase it from him. However, if a new defect occurred with the second buyer and he kept the contract, he has the right to claim compensation for the old defect from his seller, and then that seller can claim it from the first seller.