Is a guarantee required when purchasing a drink at a price three riyals lower than usual, knowing that this price is specific to a certain sandwich?

It is not necessary to guarantee the price difference based merely on doubt, as the default is the acquittal of liability and that the seller made no error in determining the price. The difference in price between applications is not proof of error, especially if the price is specific to a certain food item. If you wish to be cautious by referring to those in charge of the program to ascertain the correctness of the price, then this is good.