I am sorry but I think you're fighting a losing battle. It's highly unlikely you will be able to contact German customer support. eBay in general tend to side with buyers and their return system is more or less automatic.
If the buyer escalates their case ebay will almost certainly enforce the return or may even force a refund without return. That would also harm your account.
So the best thing is to accept the return. Get the item back and then issue the refund. Put the buyer's ID on your blocked bidder list.