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Question 1: You are only required to ensure that their total pay divided by their total number of hours (40 or less) is at least minimum wage. You don't have to ensure that each hour itself they earn minimum wage, so they could earn $0 for some of their worked hours, as long as their overall pay makes up for it later on. Ultimately, the total pay though must be at least minimum wage. You could, however, choose to pay them a minimum wage while they are working hours that they are not cutting hair and require them to do other tasks. That's actually very common and there is certainly nothing wrong with that.
Question 2: You can't reduce their pay after they have worked for it, unless it is for safety violations. The laws a pretty strict on reducing pay for time already worked and it's so hard to do, it's not worth the effort. Instead, you could reduce their pay for coming hours. So, you see violations and they haven't complied after a warning, you bring them in and say " because of your violations, next pay period's commissions will be reduced by 10%." They can then either choose to work under those circumstances or quit, and forfeit unemployment based on resigning without good cause.
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