What is the ruling on the aforementioned act that achieves a material benefit for the waiter?

If the questioner receives invoices in dollars and pays them in local currency to the coffee shop, this is not permissible for him except with the permission of the coffee shop owner. This is because he is a specific employee (ajir khass) and an agent for the owner, and he may only act within the scope of his agency. Furthermore, currency exchange is a sale, and an agent is not permitted to sell to himself without the permission of his principal. This is the view of the majority of jurists.