Not able to download Server App because it's registered with another Apple-ID
I bought a used Mac Mini Server with OSX Mountain Lion Server, but the Server App is obviously registered with the Apple-ID of the former user, and I'm not able to download it even after a fresh installation of the OS.
Is there a way to transfer the App from one ID to the other? Or will I have to buy the whole Server Update again although it should be bound to the system?
Mac mini Server-OTHER, OS X Server