The agent abiding by the price fixed by the authorizer
If a person authorizes you to sell a commodity for him and stipulated the selling price, (Part No. 14; Page No. 274) then you sell it for a higher price than what he stated, the extra amount will be the right of the owner of the commodity except if the owner permits you to take the amount. In this case, it is permissible for you to take this amount. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.