MTN agent makes a mistake and I must pay

MTN at The V&A Waterfront made a mistake with my data purchase. Now have to buy more data to get me through the month and it’ll cost me more money. I wanted to buy a month package and they sold me a week package. Even though I won’t use all the data in the week, after 1 week my unused data disappears and I’ll be out of pocket. It’s never in my favor. Any mistakes are always in MTN’s favor. Very unfair. And there is no recourse!