L. Sprague deCamp 
Fallible Fiend [EPUB ebook] 

The Perfect Servant (NOT!)He looked like a cross between a dragon and a catfish, and he could bend iron bands into pretzels with a flick of his hand. But what Zdim the mild-mannered demon really was, was a scholar of logic and philosophy. That’s why when Zdim was drafter for a year’s servitude on the mortal plane he felt that a monumental administrative error had been made.And even though Zdim resolved to be absolutely obedient and to do exactly what he was told, the wizard who employed him soon agreed.