You have the right to work there or not.
Here is what I recommend you do. I would e-mail or in writing inform your boss of your agreement and that you are not able to work any additional hours. Keep a copy for your self. (don't go to the owners yet this will make a tuff situation with your boss)
Then if you are issued more hours than you can work send him a letter or e-mail reminding him you cant work it.
If you get fired you will have a shot at unemployment while you look for another job. If you are fired that will be the time to go to HR or the owners with your letters or e-mails to your boss about your agreement. (who cares if he gets mad now)
The owners may look at it in a different light if you are fighting to keep working for them rather than complaining about your boss.
Good luck to ya.
