I don't know about Amazon Germany, but the gift cards from Amazon Japan aren't transferrable to Amazon US, nor vice-versa. I just suck it up and purchase Amazon US gift cards in e.g. $200 blocks with my Japanese credit card, incurring the exchange fees only once. Then I just buy all my Kindle books with those points.
Another idea might be to find someone in the US (a German expat?) who wants Amazon.de credit, and do periodic gift-card exchanges?
