ACS Education Services Student Loan Complaint

ACS Education Services Student loan department,

Non-federal student loan Dealing with my lender or servicer California

I was a co-maker on the student loan with my son, - -. In - 2015, I refinanced my home to pay of this loan in the amount of $97000.00. On -/-/2015, the loan statement sent by ACS gave a loan balance of $96.00, -. I made a payment on -/-/2015 in the amount of $97.00, - on this loan having refinanced my home. I now have received several calls ( - 2015 and into - -, 2015 ) and bills dated -/-/15 ) stating I owe $530.00. In my many conversations with the representatives, I have asked for a detailed explanation of why I still owe this amount. I am very confused because the amount due on -/-/15 was $96000.00 and payment of the loan through escrow on -/-/15 was $97000.00. Please help!!