Working out the price of the car after ascertaining how the client wishes to pay

Q: I am a car dealer. I am not involved in finance. However, I accept different forms of payment, such as cash, EFT and via a card machine. Is it permissible for me to decide the price of a vehicle depending on the customer’s form of payment? The bank charges me 3,25% (for a credit card) and 1,75% (for a debit card) of the total amount if the card machine is used. Can I add this percentage onto the original price of the vehicle? If so, does it have to be the exact amount the bank charges me or can I round off or add to the amount as I desire?

Error in the supplier’s calculation

Q: I do accounts for a certain person. Recently, a situation arose where according to my books, we owed a supplier R100 (for example), but according to the supplier, we only owed him R50. The reason for the difference is that the supplier made a mistake in his calculation. I pointed this out to my employer but he still chose to remain quiet. What should I do?