Is it permissible to work as a commercial broker in non-Muslim countries and receive a percentage commission from the buyer, with the possibility of adding this commission to the price of the goods without informing the buyer of its amount?

It is impermissible to add a commission to the price of a commodity and present it to the merchant without his knowledge of its amount. Contracts with merchants are not valid unless the amount of the commission is specified or it is a percentage of a known amount.