I am a truck driver for a moving company. We have one account that regularly takes 12 hours to complete and most of the time it is on Sunday, which would be overtime. I used to just punch in and do it as a normal day of work, but about a month ago, my employer said I could no longer do it on the clock, they were going to flat rate me at 100 dollars for the day, which is a huge pay cut. They told me to take it or leave it, so I told them I couldn't work for that much money. But now they won't stop asking me to do it, they keep trying to pressure me into it and I don't like it. They have also asked me to work for free, cause they said they didn't have money to pay me. Are they breaking any laws?

They may change your rate of pay for all your pay, Just because it is on Sunday, does not mean it is over time, it has to do with total hours of the week, not what day of the week it is on

I guess they can "ask you" to do it free, they can't make you do it free.