Paid debt settlement but now being sued

I entered a debt settlement with Chase bank. The settlement was for three payments. Chase has record of the first two payments, however, says they did not receive the 3rd and final payment. I made the third payment 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. I called numerous times, sent a letter with my bank statement. 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. The transaction happened on the phone. I asked if it was possible if they credited someone else's account, however, they just kept sending me to different representatives and continued to say they never received payment. This is frustrating because I made the payments on the dates as agreed and my bank statements verify this. Do I really need to hire a lawyer and spend more money to fight something I already paid? 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!
Thanks for your help.