And the Devil Will Drag You Under — Jack L. Chalker

Asmodeus Mogart was not a bad fellow, as demons go. Having gotten in trouble back in the home office, he had been assigned to duty on Earth. There he toiled, doing the kinds of things demons do and turning into something of a drunk.
Then a rogue asteroid threatened to crash into Earth and destroy all life on the planet -- demons included! There had to be a better way.
Mac Walters and Jill McCullough, holding a private wake for their world in a Reno bar, were more than startled when a strange-looking little drunk told them they could save the world.
All they had to do was enter five alternate universes and steal a demon-guarded jewel in each. Clearly, the man was crazy.
But they had nothing better to do than go along with the gag. Then they each found themselves, naked and alone, on a hostile alien world!