To Hex with Sex (1969)

Sinopsis

Marvin Swift is a nebbishy schmuck who’s "a total failure at everything." Fired from his job at a brassiere factory, he then gets into a pointless argument with his sexy but ditzy fiancee who tells him to go to hell. Instead, he does the next best thing. He meets the devil in a boiler room. And the devil’s a she. A very sexy she named Lucibel sans the traditional horns and pointy tail because "that’s terribly passé, today its mod." Touched that Marvin is such a screw-up, she offers to help him but not, she says, because she wants his soul, but because she wants to make him happy. So she grants him a wish with no strings attached.