Can an employer refuse an employees use of PTO hours past a 40 hour mark?

Question Details:

I was sick for 3 days last week. I put in a request for 24 hours of vacation time. The payroll person only put me in for 16 hours. HR said that it costs the company too much money to allow us over 40 hours unless we've actually worked over 40 hours. Basically, they are telling us when we can and cannot use our earned time. As of right now, I have over 100 hours of PTO/Vacation, but I was only allowed to use 16 hours. Is this legal? Important Notice