A creditor said I had to pay $450/month on a debt I was behind in.I can pay $225/month.Can they sue?

I live in Tennessee. I have corresponded with them from the beginning and have told them that now I can pay the $225 I was paying before I had problems. I have paid payments of $500 once and $450 three times. They insist I have to pay the $450 until the card is paid off. Is this legal?