All four versions are correct: басурман, басурманин, бусурман, бусурманин. In Dahl's dictionary you can see even the fifth one: бусорман (!).Originally Posted by Yul'a
There's no point in arguing over spelling, when the word itself is a distorted and colloquial archaic version of the 'foreign' word Muslim ("мусульман"/"мусульманин").