Is my job as a sales representative permissible, given that I collect small differences from the value of the merchandise to sell it to another client at a lower price, and sometimes I add from my own money to complete the sale?

The sales representative must adhere to the limits of the agency and not exceed the company's authorization in their actions. If a selling price is set for them, they must abide by it because they do not have the authority to act beyond what their principal has permitted. They must also refer back to the company and inquire about any ambiguous actions before proceeding with them.