Discovery is a ****

I had a terrible experience i was booked for a operation then had to get a reveral with a authorisation number which i did receive from discovery via email the 05/28 Which confirmed it was covered by the medical aid which i proceeded to go through with the op on the 30/05 which was successful and a week later when i received my bill and Netcare contacted me they had told me that discovery had pulled back the authorisation which is over 8000 rand and more than my monthly earnings that i can not afford to pay . Why give the authorisation and then pull it back? I would’ve never then went for the surgery if I knew this.