Promotional SIM card data saga!!!

I took out a contract with vodacom last year May & this contract came with an additional SIM card. When I inquired at the store they said it’s a promotional SIM card which comes with 1Gb data which I WONT pay for, & it’s for the duration of my contract. In December now they send an email saying that it’s a billing error on their end and they will start billing me for the additional data ( which was promotional & which I did not as for ),which they did do. I call the call center to cancel this as I did not request it and I did not ask or agree to it to begin with ,I was told that I cannot do so ,if I do I will be charged for early cancellation ,and and and…. My issue is this ,I took out a contract for 1 plan which was not a combo plan with a promotional what not but now Vodacom is making their mistake MY PROBLEM and is charging me for something I did not agree on or request. I need this removed from my account and I want my monthly premiums to be what I had agreed on and nothing more.