Charity that cannot get a refund

We are an NPO in South Africa and we booked a car and flight for a VIP guest on 28 Jan. When the guest got to the AVIS counter, they told us we owed an additional R2500 which we were never informed of. As far as we knew we had paid in full. As a result he could not get the car! We even offered to pay over the phone with a credit card and they said no. Needless to say we were highly embarrassed having inconvenienced someone who is a celebrity and agreed to do us a favor! We got him an Uber and didn't use the car. I am now trying to get the refund for the car and all I am getting is a pass on from pillar to post - its not BA, you must contact AVIS.....its not AVIS you must contact BA holidays.....and so it goes! I have put 2 requests for refunds on their site and nothing. For this NPO every cent counts and we are getting nowhere! Please help!