A PROPER demon lord would have Sam killed off immediately before he gains too much power. Maou would probably shoot the shit and be jealous that Sam has a bigger apartment (Since he doesn't live in Japan). He might EVENTUALLY get around to trying to deal with the competition, but it's not a high priority.

Sam, on the other hand, would be quietly freaking out in confusion. This guy is a demon lord!? But he ALSO works at McDonalds?