Can you force comission employees to work OT?

I work for a debt consolidation agency where I was originally hired as a " comission only " employee. Due to the fact that some weeks, there are some employees that DO NOT acquire any deals the employer enacted a draw vs comission pay plan to cover himself against not paying an employee at least minimum wage on a weekly basis. Now my comission paychecks haven't fallen below $500.00 after taxes any week with this company since I began there so it doesn't affect me, but I believe he also initiated this new pay scale to insure productivity and " weed " out the non-producers. In doing so, he also implemented mandatory Saturdays which is not the premise I was hired under. Now on average Monday thru Friday I put in an average of 46-53 hours a week. So I guess my question is can an employer who is basically paying you by commision only force you to work overtime, can he change the original premise of voluntary Saturdays, and can he fire you for not working those Saturdays? If so do I have any recourse?