Transporter borrowed accident who pays?

Hello, I need your help once. I borrowed a transporter today to pick up garden furniture. On the way back I stupidly drove off the exterior mirror of a parked sprinter. No damage was caused to the rental vehicle. The damage was then taken up by the friendly helper and back to the rental company it was then I had to pay the damage with the excess cost of 500 euros itself. To my knowledge is the partial cascade for the rental vehicle and the liability insurance is liable in case of damage to the external vehicle. For the liability there is no excess in the contract so I would have to be out of the matter without paying anything or? I would like to thank you for your answers