I don't mean to sound rude, which I probably will..., but .de = Germany, which is in Europe, hence they gave you an EU key. So that company hasn't really done anything wrong, they supplied a key for their region.

Unfortunately SE never swap keys for different regions or even change peoples regions even if they move country. So I'm afraid all I can really suggest is try reselling the key yourself and/or trading with someone else that has the region you need and they need yours. Of course there are risks involved with doing that, but that's up to you to decide if you want to try.

Sorry I don't have a better reply.