Unauthorized debit order

I was on a month to month wifi contract R269, stopped using their service on October 2024. Got a invoice in 6 January 2025 to pay the amount R269 but got a MTN Debit Order failed message of R2839 on the 4th of February 2025. I called MTN and I was advise to return the device so the billing can be recalculated of which I did on the 10th of February but now MTN consultants are telling me that I can't cancel the contract before I pay the R2839, but why should I pay so much while I returned the device?????