Unhappy

An agent contacted me to reactivate my account and at a monthly fee of R495 because I have an active medical aid on the 4th of October and I signed the contract. Yesterday, the agent contacted me to say they have to change the amount to more than R550 because they cant activate my medical aid. I sent him proof that if that's the case, I have to cancel because I cannot afford the monthly instalment, and he threatened to say he would charge me 40% for the yearly amount to cancel. I tried to explain to him that the reason why I am cancelling is that he wants me to agree to something that I did not sign for. Why is he able to change the contract and I am not allowed to say no to something I did not sign for