Employer not compensating staff for extra time

Q: I work as a manager for a Muslim man who at times stretches the time of his employees if it gets busy or is attending to personal work. He does not compensate them for extra time, and when the topic is brought up, he angrily dismisses it and says, "You are just standing doing nothing, so it’s not work." But I feel as long as they are in the shop, they are working, as they are not free to do as they like. What should I do ?