Upgrade with Mtn nightmare

If I could have given a zero rating I would. I was contacted by a salesperson and offered a very attractive upgrade on my existing data package from Mtn. Apparently, I would be saving money as this would have cost about R100 less. So when it was time to pay my bill, I EFT'd slightly less than normal. I then received an sms from Mtn informing me I was R715 short!! And now I'm being sent from pillar to post. Can't get hold of Summer mobile and Mtn can't offer assistance. I've been a loyal customer for over 25 years with Mtn, but this seems to have very little significance to Mtn.