If you are a salaried exempt employee, your salary covers all hours worked no matter how many or how few. You are NOT being paid for 40 hours with all other time being unpaid.
There are no circumstances whatsoever in which an exempt employee is entitled by law to a single penny over and above their regular salary, no matter how many hours he works.
All employees, regardless of exempt status, can be required to work as many or as few hours as the employer wants, and can be terminated if they refuse. The only question is whether or not they are entitled to additional compensation. If you are exempt, the answer is no, you are not.
And before you tell me how your paystub says you are only being paid for 40 hours, I will tell you that most if not all payroll softwares have that as a default that prints out on all exempt employees - they have to fill the space with something so they fill it with a 40. The LAW is as stated above.