Tax Invoice with big discount - But didnt get it

I recently bought a car from McCarthy Renault Woodmead. During the process of finalising the deal. I obtained a TAX INVOICE with a R82 179.83 Discount. Surprised and happy about the huge discount. I asked about the discount. I was told it is not binding due to the fact that was not \System Generated".<br> <br> The document said it is a TAX INVOICE.<br> <br> Documents stating they are tax invoices are legally binding. Regardless there origin.<br> Even hand written tax invoices are legally binding """