citifinancial misleading info

Hello thank you for any advice. We had an account with citifinancial for some flooring in our home. It was a 12 month same as cash. Prior to the 12 month time period, I contacted the branch office and asked what my options were as far as pay off. The employee said that I could make payments on the account in the amount specified on the statement and then when I paid in full I could pay the statement amount. I paid this account in full 4 months after the 12 month same as cash due date, paid over $270.00 in interest in 4 months, with a check marked "paid in full" which they cashed. I am now being billed for over $500.00 in additional interest due to the fact I paid after the 12 month loan period. What options do I have? I feel they mislead me with the advice I was given. Thanks for any help

[bigun]

What does the orginal contract say? That's the important thing.