Receipt error and initial overcharge

2014-11-04 pay R2500 upfront for gastroscopy, receive receipt 12837<br> 2015-02-03 get SMS demanding payment of further R883.39, request and receive account statement<br> 2015-02-09 request full statement showing R2500 paid<br> 2015-02-17 resend request for full statement<br> 2015-03-05 get another SMS reminder and the same statement of account<br> 2015-03-06 After complaining by telehone, it is acknowledged that the receipt was indeed incorrectly recorded and I was R966.61 in credit.<br> Two issues:<br> 1. Why overcharge the patient on an upfront payment?<br> 2. Who records receipt 12837 for R650 if it was issued for R2500 - how did they balance for the day?