Gateway to Darkness — Fredric Brown

Crag was a no good drunk and deviant, now facing life in prison. But the good judge Jon Olliver offers him his freedom -- plus a million credits! -- in exchange for a small favor: steal a harmless tool from a rich scientist in Mars. Crag suspects a catch, but he finds it so hard to resist … especially when he knows that Olliver's beautiful wife, Evadne, will be joining them in the adventure!

A great 1940s novella from a master of the form.