Should the heirs give in charity the amount paid by the buyer of the apartment 20 years ago, or the amount for which the apartment was sold recently, knowing that they do not know the relatives of the buyer?

If the sale was concluded between the grandfather and the buyer, then the heirs' sale of the apartment without the owner's permission is invalid, and their action is null and void. They are obligated to return the price to the one to whom they sold the apartment. They are not permitted to give the price in charity on behalf of the apartment's owner, nor can they annul the sale that the grandfather concluded. They must return the price to the one to whom they sold the apartment because it is not their property, and they should leave it for its owner, or they should refer the matter to the judiciary.