Is it permissible for an employee in a marketing company to receive a commission from increasing the selling price of products beyond the price set by the company, in addition to his salary?

If the company authorizes an increase over the specified price and allows you to benefit from the surplus, then there is no harm in that for you. It has been reported from Ibn Abbas and Al-Zuhri that this is permissible. Similarly, in Al-Mughni: "If a person hands over a garment to another and says: 'Sell it for such and such, and whatever you add to it is yours,' then it is valid." And it does not matter that you are an employee of the company and receive a salary, for what the company has authorized, there is no harm in you taking it.