Unfair penalty for upgrading a data contract

I had a data contract for R199 for approximately 4 months. I kept running out of data so I went to Telkom to upgrade my contract to a 10G plus 10G for R299. The shop assistant was very slow and kept complaining about how tired and bored he was. I asked if there would be a pro rata amount that I would still have to pay for the previous contract and he said yes, but I was never once told that there would be a penalty by the shop assistant, and now I see on my account that I am being charged R945.19 to upgrade! I don't understand how Telkom can charge me to upgrade to a contract that is more expensive?! This is ****************! If the shop assistant had told me upfront about the penalty, I would never have upgraded my contract because I cannot afford to pay the penalty. I am very disappointed and disgusted with this service and I am stunned at the sneaky way in which Telkom handles customer contracts. Telkom should ensure that all costs should be made clear to the customer upfront before a transaction takes place so that the customer can make an informed decision. If this is how Telkom treats loyal customers, I will take my business elsewhere.