Bill issue

On the 3rd of February I was due for an upgrade, went into the store to enquire about upgrading but they didn't have the phone I wanted in stock, I was told I would get a call once they have the phone in stock. I got a call on the 10th of February, went into the store and did my upgrade, and I was told that I would not need to pay anything for upgrading. My bill for February came, it amounted to R508, of which I'm OK with since I upgraded 2 days before I get my allowance (data, minutes) and I made that payment. Then my bill for March came amounting to R989 which is incorrect, my upgrade amounts to R499. I went into the store to enquire about the March bill, it came to my attention that you have added a Device amount from my previous upgrade... I certainly not happy about this, now you have blocked my phone but there is a pending investigation... I'll be definitely canceling my contract with you. This is unacceptable!! This needs to be fixed ASAP as I'm using my phone for school purposes