In Germany, bounties are relatively strictly regulated by law in terms of what you may and may not do etc.
Therefore, if the bounty itself was valid in the sense of the law, then - AFAIK - there's an obligation to pay up.
Anyhow, while IANAL, bounties are regulated by German law in (among others) BGB §657-661a. Mostly §661 and §661a. You can find a translation of those paragraphs here: http://www.gesetze-im-internet.de/englisch_bgb/englisch_bgb....