If the factory-owner or shopkeeper gives you part of the money from each product that is sold through you, in payment for your efforts to find him customers, and this money is not added to the price of the product and that does not cause problems to others who sell the same product, and this factory or store sells it for the same price as others, then this is permissible and there is nothing wrong with it.

 But if this money that you take from the owner of the factory or store is added to the price paid by the purchaser, it is not permissible for you to take it and it is not permissible for the vendor to do that, because this is harming the purchaser by making him pay a higher price.

 And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions.