Originally Posted by
SteamyUncle
You could park your German car in Vienna, pick up another car, go to Bratislava and be back in Vienna 3 hours later to drop the Austrian car and pick up the German one.
It may be the best solution. ^
You can also check how much will cost get a car in Germany, at Austria drop it and get a new one, to finally drop the new one at Germany. Less hassle, but I believe more expensive.