Licensed car sold to family abroad – how to unsubscribe?

I have sold a registered car (without license plates) to my family abroad and wonder now how I can unsubscribe here. The registration office has kept all German documents (both certificates) … they are now automatically sent to Germany and my car is logged out here or is the German registration office waiting for me to appear and that I still submit the registration plates and the purchase contract? Does anyone know how this works?