Is it obligatory for someone whom the shop owner gave 40 riyals instead of 50 riyals to return the increase?

If you did not lie to the seller, then there is no blame upon you for the price reduction that occurred with his consent. However, if you lied to him, then that is not permissible for you. In this case, you must return to the seller and inform him of the true situation. If he is content with what he took, then there is no blame upon you. But if he is not content, then you must pay the remaining price.