The federal road tax is paid when the fuel leaves the bulk distribution center, ie. at the rack. It is included in the basic price that the wholesaler pays for the fuel. The tax has already been paid by the time the undyed fuel gets to the station. All fuel that has NOT been federally taxed MUST be dyed red before it can be transported from the distribution center. That is a federal law.
If the station owner is rebating you the federal tax from non-dyed fuel, then he is loosing money.
Now, state tax collection may be different depending on what state you are located in. My guess is that he is rebating the state portion of the fuel tax. The state fuel tax amount varies from state to state as do their collection rules. Your state may permit rebating at the pump, while mine doesn't.