The shipper can call who ever they want to ship. I would guess it depends on what sort of deal YOU have with the broker. The broker will be the one mad at the situation, and could make it difficult for YOU. The broker may boycott the shipper for doing it, but the shipper won't care, cause they can call anyone. So if you depend on work from the broker, better re direct that call. The shipper is normally just trying to jump the list and get his stuff shipped first. ITs not about money. I'm not sure I would want to be apart of that, just bad karma and gets you a bad name. For what? One delivery? If the shipper offered you a contract with full time work, then it would be worth it, and wouldn't be breaking any karma or rules as such.