Discovery not fair to members

My son was admitted to hospital for surgery. Full authorization was obtained from Discovery Medical Aid for Mediclinic Limpopo - Authorization No. ***3. . The operation was done as per the authorization and the hospital was paid by the medical aid. A month later the medical aid phone and wrote to my son that he has to make a co payment of R5500 and they reversed this amount from the hospital. The hospital today sent an account for R5500 as the medical reversed the payment. I believe Discovery are not fair and as a consumer of any service, once a service is authorized by the medical aid they cannot change a month later and request a co payment. A person has to be timeously informed and not approve first and then not pay even though they authorized payment