I have a major problem with Citibank. I sent a billing error notice to the correct office, and never heard back from them, even after reminding them 60 days later. I sent an accord and satisfaction letter back in May for about 20% of the debt, and they cashed it (I sent it to the same office as my dispute letter, and they forwarded it to the payment center). I thought that was the end of it. Today, 6 months later, I am being sued for almost double what I originally disputed. What can I do?