Since they told you from the beginning that they would file suit against you regardless, there is not much of a defense. Depending on the circumstances, you could argue that the payment arrangement constituted an accord and satisfaction. Alternatively, if you are having other financial difficulties, a chapter 13 case could make sense for you. You could have up to 60 months to repay this debt, and many other debts as well. Definitely talk to a qualified attorney to see what the best option is.