Incorrect contract details entered

I went to Istore in Cavendish. I requested a contract for the U Choose Smart M which the salesman,Grant, confirmed.For the first few months the correct amount was deducted, now for the last 2 months the account has been chargedwith unlimited data as a Smart M account. I went to the consultant at Canal Walk, Thembisa Dolo on 6 April 2016 to have the contract migrated. On 17 April I received my latest account to notice that I have been charged R1084.29 for data. This is not what was agreed originally. I wanted a closed account with a limit on spending and this was confirmed at the time of signing the contract. And yet the contract states Smart M. Only now have I found out that Smart M means unlimited spending!!!!! I want to be refunded for all the additional costs to which I did not agree.