That's really strange. I got this key from Humble Bundle, so it have no specific region. It seems that Steam itself blocked the game for Germany (you can check if Sniper Elite is in the Steam Store in your region), but some time ago I've activated a lot of products that are not available in my country using the keys from international sellers.
I would advise you to contact the Steam support and ask them about this. If it turns out that Steam blocked activation for the keys of the restricted-in-your-region products, you can keep this key and give it to any of your friends from other countries OR just say me that you refuse it and I'll request another winner for this giveaway. Just let me know.