Unfair Final Billing After Timely Contract Cancellation

I am extremely frustrated with MTN regarding my contract cancellation. My contract ended in November, and I canceled on time, giving the required 20 days’ notice. My SIM card was successfully transferred to a prepaid plan, and I have been paying my monthly fee of R580 without issues.

I only just noticed on my statement that I was charged R1000 on the last day of my contract. I usually pay my account in person, so when I went to pay the final month, I was told I owed R600, which I paid immediately. Now received a statement that I owed R1000 and paid 600 and I have an outstanding amount of R433, which I need to pay now or my account will be suspended.

I don’t understand why the last payment was R1000. And I was told of R600 when I went and pay.