Authorisation - Don`t want to pay

My daughter was submitted to hospital on the 23/11/2015, she is only 12 and was complaining of lower back pain. The doctor ordered x- rays, and then said that he want to do a MRI scan. Discovery medical aid gave the x- ray department (Dr`s) an authorisation number to do the MRI, NOW THEY DONT WANT TO PAY, BECAUSE IT IS NOT PART OF OUR PLAN. How is it then possible to get a Authorisation number in the first place?<br> <br> If you give a authorisation number you authorise that procedure, what is the use then of an authorisation number if you can the go back on your word???