Vodacom Retention Department Disappointing service

I called Vodacom on the 16th of May to request a premature cancellation settlement amount. On the 17th of May I sent the documentation and proof of payment. I received an email stating that the premature cancellation had been processed. Last week I receive those programed calls informing me that an amount of 659.00 is outstanding. Today I called and I'm informed by Nomthandazo Msimango at the Retentions Department that the agent helping me on the day was supposed to inform me that the May invoice had not been considered which she didn't and I settled the account. I refuse to pay this money for a mistake a Vodacom employee did. Nomthandazo Msimango after consulting with back office informed there's nothing that can be done and that they will refer the matter to the agent that assisted me to escalate. Imagine, I told that at this moment, she's the one representing Vodacom, she needs to speak with the authorising manager and revert back to me. She told me that she wasn't going to lie to me that my suggested approach would not help..disgusting if this is the callibre of customer service Vodacom instills.

I need my account fixed asap as I am not at fault..