What is the correct action after discovering that the purchased phone's memory is 128 GB and not 64 GB, taking into account the change in prices between the date of purchase and the date of discovery?

If a person buys something and then discovers that it is more than what was agreed upon with the seller, the surplus is the seller's right and must be returned to him if possible. If it is not possible to return it, the buyer may purchase it from the seller and pay its price. This is what jurists like Al-Bahuti and Ibn Uthaymeen affirmed, stating that the surplus belongs to the seller. They also mentioned that both parties may annul the sale to prevent the harm of partnership, or agree to uphold the entire sale with compensation for the surplus. If it is impossible to return the surplus (as in the case of a phone with additional internal specifications), an agreement must be reached with the seller on a sum for that surplus, estimated based on the item's true value and specifications at the time of purchase. If they disagree and do not reach an agreement, either party has the right to annul the sale, whereupon the seller reclaims the item and the buyer reclaims the price.