Unprofessional and unacceptable.

My mother had to have a procedure done. Dr. Steyn's office called me to confirm that the medical aid will cover the procedure in full. 3 months later we receive an account for R2,827.70. I went to their office today and the lady dealing with the account, tells me that she is very sorry, they made a mistake with their initial calculations. She can't help me, I should try and arrange to pay the amount still due in installments with the debt collectors. This is unprofessional and unacceptable.