want to cancel contract and then get huge invoice

In November I send a cancellation letter to Telkom (during my upgrade time), because I am unable to upgrade to a data deal only. I then took out a new contract because I wanted data only and then I would cancel this one. I called them again in December to find out when this will be cancelled and they said that someone from the retension unit will call. I explained to them that i took out a new contract to cater for my needs at Telkom (which is data only) Today I received my invoice and I now need to pay R594.89. Usually its R109. How is this possible? I called and they said that I must call Monday 4 Jan because they did not receive the invoice yet. this is really crazy. What must I do? If they deduct this money?