DEBIT ORDER DEDUCTED NOT WHAT WE AGREED ON

I tried cancelling the tracking unit in May 2015 as I sold the vehicle it was installed in. They failed to cancel the device and in July I finally got a cancellation confirmation. They then advised that the cancellation settlement will be R395.83 to be deducted end July. They failed to debit this amount, only deducted normal instalment. This month, they deducted R520.66 as the cancellation settlement. I am very unhappy about this, as we didn't agree on this amount and the fact that the cancellation dragged on so long!! I still have all my emails between myself and Natasha re cancellation and settlement amount and will take this further, should I not receive a response! Mrs MM Van Heerden