Paid debt settlement but now being sued??????

I entered a debt settlement agreement with Chase bank. The settlement was for three payments to satisfy the debt. Chase has record of the first two payments, however, says they did not receive the 3rd and final payment. I made the third payment via telephone on the date as agreed and have my bank statement showing it was taken from my account. However, what was taken from my account was $300 more than what Chase agreed in writing. When I realized what had happened, I called numerous times, sent a letter with my bank statement but no department could help. They kept saying the did not receive the final debt settlement payment. They sent my case to a lawyer whom I also sent confirmation of payment. My case went to arbitration. This is where I made a mistake. I sent my response refuting the debt to their lawyer and not arbitration. I lost. I recently received a summons to appear in court to uphold the arbitration award. I don't know what Chase did with the final payment and I paid more than what was agreed. I have my bank statement with noting the withdrawal to Chase on the day as agreed in the debt settlement. Do I really need to hire a lawyer or is having my bank statement sufficient? I don't know what more I can show except for my bank statement and letters I sent to both Chase and their lawyer to help clear up their mistake. What should I expect at judgment confirmation? It seems unfair that I have proof of payment (actually more than agreed) and yet now they are suing for almost twice the amount! I don't want a judgment filed against me and I don't know what to do.
Thanks for your help.