Different prices at a gas station

Moin, there I drive yesterday afternoon to the local Bft gas station, because the price for super gasoline was attractive. 1,379 € was a favorable price for our region. Otherwise I prefer to use a HEM gas station in the commercial area. I drive to the pump and refuel. During the process I notice that the price did not correspond to the price tag. Here I demanded 1,439 €. I interrupted the tank process immediately, but I had already for 43 € fuel in the tank. The man at the cash desk drew out that I had filled during a price change and the conversion time needed. The price indication at the pump would be relevant. Can that be? I would not have hit the gas station otherwise at all. Is this a fraud case? Presence of false facts? Many greetings Thomas