Dishonest billing for services rendered

The radiologists did a scan on both my ankles, before they did the scan they told me I should pay R2400 upfront and the medical aid would cover the balance(discovery), I paid the monies and the procedure was done, months later I was shocked when the radiologist send me a letter of demand for a further R3000 through their lawyers,I feel it was very unfair to come with additional charges , if I knew that information beforehand I could have shopped around, or abandoned the procedure altogether , please help