Unauthorized payment using my on file debit card

I canceled my service with xfinity and then a couple weeks later my final bill was paid using my card without my consent or authorization. Went into the store and the associate could see on my account that an xfinity agent "assisted" with making this payment, again without my authorization

 I was then directed to another agent who looked at my account, same thing, someone authorized a payment with my card without me consenting. I was then directed to call xfinity security department. I called them talked for a bit and was directed to an account manager. They investigated the unauthorized payment and told me the only way they can take a payment is from a customer authorization, which they did not recieve from me. Some "agent" assisted with manually making this payment with my card and they told me that it was impossible without me authorizing it. I immediately think this is straight illegal and some sort of fraud on their end. The account manager tells me that I have to call the bank and ask them why they let this payment go through. And if I get my money back I can call them and make a payment. They told me it wasn't possible for them to make the payment without authorization. Is it really impossible to override the authorization on their end? I doubt it. I'm very concerned for the fraud of this company on a long standing customer and hope to get this resolved before I have to take some legal action.