Sounds like they did not have the replacement manager till 9/9, so had you work in his/her place?
The contract from 7am to 5 pm is only 10 hours, or 9, if you have a 1 hour lunch break. But, working 6 days would equal 60 hours or, 54 hours if you have a lunch. Granted with salary, you get paid the same each week, if you work 40 or 80 hours, and 3 days or 7 days.
You claim to be on hourly, so what was your hourly wage? If you was working 6 days a week and even for 8 hours each, then you would be working more than 40 and would be owed overtime.
Like the other stated, to sue, you can only claim what you lost? So, my guess would be only the difference of your prior hourly wages vs the salary paid.
Also, since new manager came in after a week, if you had not done the job, who would have? In other words, would your boss been stuck doing it vs you?
You need out of there asap. Start looking for another job in your same position. Who is your competition? That is where to apply, since competitors love to steal employees from other competitors.
Then (only) after you leave, go to the wage/hour division in your state to see if they owe you for any past wages.