change of booking - Ref - MLJWKG

On attempting to change a flight on the website, instead of the R280 charged for a change, the amout due was reflecting as R1893.73. This despite the fact that the flight I wanted to change to was cheaper than the original flight. I didn't take the transaction any further, as I realised there was an error on the system. I phoned both the Booking call centre and Customer Care and was told they could not assist me as the change was already reflected on the system, and showing the amount due as R1893.73, and I should send an email. I send this to the guestcare address I was given and have received no response. I then also forwarded to the enquiries email address, but still no assistance. I must add that the original flight cost R872 and the changed flight R752. I could book a complete return flght for less than the supposed charge for the changed flight and still have some spending money!! Ridiculous! Assistance please!