Vehicle cash price changed

I recently bought a car from McCarthy Toyota Bruma. Service was excellent but after signing the contract I realized I was charged R490 more on the vehicle. I immediately phoned the person who assisted me in the sale. And he apologized and said I will be reimbursed for the extra amount...He suggested fuel which I declined and suggested the monies be paid to me or to the vehicle account. He said the vehicle account was not yet active and as soon as I pay my first installment I should phone them (the salesman and or the finance assistant whom assisted me with the finance application)to transfer the monies to that account....When the first installment went off I phoned my bank first to find out about the extra registered amount and they informed me about the interest thats also included which McCarthy should work out in that amount as well and pay to me(Mfc also suggested I can report them to the bank (Mfc)if this is not resolved)..... I then phoned McCarthy Just to find out he is on leave and the finance person has no knowledge of my complaint nor the agreed on solution....This is whats happens when you a first time buyer and sales people take advantage