The source for the idea of selling your soul to the devil for a werewolf belt may be this story:
Another unfortunate werewolf was Peter Stubb of Cologne, tortured until he confessed having transformed himself into a wolf by a magic girdle given him by the devil. The judges couldn’t find the girdle where Stubb said he hid it, but they explained this by saying it had “gone to the Devil whence it came, so that it was not to be found.” Though his case was unproved, Stubb was nastily executed for the crime of lycanthropy: he was sentenced to have the flesh pulled off his bones in ten places with red-hot pincers, then to have his legs and arms broken with a wooden axe; finally to be beheaded and burned.
From: alt.horror.werewolves FAQ.