Getting overpayment back

I took out a loan with United Cash Loans few days back. The loan was of $750 and I’ve paid them $150 over that amount. I’m here in Illinois and I’ve known that they have been operating their business illegally. I’ve discussed the issue with my bank and closed and opened another unlinked account. I’ve even sent letters to UCL and filed letters of complaints, However, UCL never did get back to me. My bank advised me to file fraud charges against them to get the money back. However, when I reached out to the fraud department, they told me that they need something in writing to do this. I’d like to know what this would be? Thanks in advance.