xfinity does not issue a refund even if they know I paid too much

Since I am going to close my account, I stopped my autopay. I paid the monthly fee for my wifi on 6/20 but I noticed that I paid it on 5/31. Since the balance became minus, I contacted xfinity immediately. I asked them to pay back but they required me a document to show the reason why I paid on 6/20 and never paid back to me. I called the customer center and the woman there told me it will be paid back and I can receive an email. However, the balance was corrected to zero and my request for refund was turned down. 

The company is [Edit: Inflammatory]. There is no conscience with this company.