You will be lucky to get out of this one, I'm sorry to say.
I did a BF3 update that was around R22000 because my uncapped bundle didn't kick in... I believe I got off because the SMS instruction from MTN wasn't clear.
You will have to wait until end of the billing month before you may launch a complaint and for them to start the investigation.
They provided NO feedback, so you will need to chase them the whole time.
First off, change your MTN payment from debit to EFT.
Then get a lawyer, or start reading up on the national credit act, consumer protection act, or any other act that may be mildly relevant.
Things to look out for is false advertisement (misinformation/omitted information/... in any and all notifications) from MTN.
Find out what the maximum allowed credit MTN may have given you, anything above that number you may be able to get out of.
Anything and everything that even smells like it might help you.
Then when the 7th come, you make the complaint and be sure to mention each and every point so that it is recorded.
If you can make 15 (mildly accurate) accusations of faults on their part, even if only 3 is spot on. That's at least 2 or 3 (maybe 4) days for whoever to work through (eish, lots of work eh?), that might just save your behind if the facts themselves don't.
GL!