Call center make mistake but I still have to pay

I called 808. On 20/01/2018. Spoke to a lady in call center. I almost do this weekly. I have 2 contracts with mtn. I ask the lady to load me on both numbers the weekly 1GB bold cost including VAT is +-R65 give or take. She loads on both numbers as requested. Few minutes later she calls me back and tell me she made a mistake and just loaded me the 1GB that's valid for 30 days @ R160. She stated she can not fix it. However all calls are recorded. She said she made a mistake. But why should I as the consumer pay R100 more.,for their mistake. I want this to be rectify. This is not about the money but the principle. I'm a business owner and when I make a mistake on a account I have to fix it. I would like this to be rectify as soon as possible.