Price Change from What is Written on the Product

I bought a product in the supermarket that had a price tag of 10 shekels, but at the checkout, they asked for 20. When I drew the seller's attention to the price tag, he immediately gave me a discount and I actually paid only 10 shekels. Is this allowed? Is there a cancellation of the sale here, since this is more than one sixth? Is this the custom of the country and the sale is valid?

Shalom and happy holidays!
According to the law, in case of a contradiction between the price that was charged and the price that is written on the product, the written price is the one that has to be paid.
However, in places where they are accustomed to following the Torah law [not necessarily according to the law], it is impossible to demand to be charged according to what is written on the product. But if the shop owner or the cashier acting on his behalf decides to refund you according to the law, there is no reason not to accept it from him.
I did not understand the issue of fraud and cancellation of sale, as what they did is to reimburse you the difference in price.