It is, if upon written request, the terms are confirmed by the one offering the bounty. This has happened here. The originally reporting newspaper has this to say (translation by me):

"[...] The doctor had noted the biologist's 100.000-Euro-Offer on the internet. At the start of the trial, he stated that he had requested (and received) written confirmation of the bet. He sent the documents to the biologist - together with his account number. Still, he did not receive the money and proceeded to sue. [...]"

Original article here: http://www.stuttgarter-nachrichten.de/inhalt.kurioser-masern...