In-hospital procedural medication not paid by discovery

My son was admitted in hospital on 23rd May 2018. While in hospital he required a Mag 3 scan. In order to perform the scan medication had to be ordered on the same day. The test could not be performed until the medication arrived since it serves as a dye or 'ink' which goes through my sons body so that the radiologist can determine the state of urine flowing through his kidney. Since that day Discovery have refused to pay for that medication which values just under R2000. Although they have paid for the procedure they have first reported to me that the invoice stated it was take home medication which i then amended with the service provider. Yesterday I received a call from the service provider regarding the outstanding amount indicating that Discovery now says that according to their rules this medication is not covered. How can that be the case if the procedure which was done in hospital would be inconclusive without the medication?