Weltman, Weinberg & Reis

I had a car repossessed and WWR filed litigation on the outstanding balance for BB&T bank. I don't deny that I owe the money. In June I made an agreement with WWR to pay $155/bi-weekly until May 2012 which will pay the balance owed in full. I agreed to a direct w/d from my checking account. I received a letter each week when my payment was w/d stating the amount in accordance with our agreement would be w/d from my account and those w/d occurred as scheduled. Then I get a call last week offering to settle for $4000. I explain that I do not have that kind of money and then they tell me that as a matter of policy they review accounts every 6 months (it has not been six months but I have made 7 payments) and this was what they were offering me. They said that if I could not pay the total amount due then they would need to increase my payment $50/mo to $360/mo ($180/bi-weekly). I just don't have the extra money right now to pay them anymore. I explained that and explained that we had an agreement and I had honored that agreement faithfully. Can they just decide to change an established agreement like that? I explained that I was told what the agreement would be by there reprsentative and the 24 month time span was given by their office and they responded that there were no notes on the account to that effect. I am concerned that if I agree to this change then in 6 more payments they are going to call me to up the payment again. Do I have any rights in this or am I just stuck?