Need last payment back after settlement

On the 2nd of Jan i requested a settlement quote, On the 7th of January I paid the settlement quote. Later that day, I received the regular invoice from Vodacom. I phoned vodacom to find out why I received an invoice and they said end of the month I would be paying another full installment.

This does not make sense as my full installment had 70% of the amount as device payments and as I had just settled the devices, I dont think this is fair. If i had paid 3 days earlier, or if Vodacom had settled my cancellation on the same day before the invoice came through I would not have had to pay this.

If i needed to pay some money for the data for the month, then fine, but not the whole amount with devices that I already settled.

Please see what can be done to credit me the device money. Thanks