Federal law on minimum payroll

I work for a large corporate retailer, and I was told that this next week that I was going to be alloted only 80 hours for my payroll.  The way my job works when it comes to payroll is that management hours are not factored into this 80 payroll (so in reality, the schedule gets made out for 120, 40 of that belonging to the manager, the remaining 80 for staff.)  My question is this: I had once been told that the federal law states that the minimum payroll hours to run for a week is 87, but my DM said it is 80.  Which is true and what should I do if the company was violating standing laws regarding payroll?