Balance Due After Pre-Auth

I had a procedure scheduled for July 2021. Before my appointment a pre-authorization was done with the Medical aid and it was confirmed that there will only be a co-payment of R2,564.00 which was paid on the day of the procedure. I also confirmed on the day that there is no other payments due. I now am receiving statements from the doctors offices with an amount outdating for R2,088.63. When I enquire about the amount I receive no reply. I paid what was due and will not pay this amount as I was assured that the co-payment was the only amount I am to pay as the Medical will cover the cost of the procedure. I would like an explanation of the amount and why it was not included in the pre-authorization.