FNB not transparent

In June of this year I used my FNB cheque card at a local merchant and payment was made via a speedpoint machine. When I received my FNB cheque account statement this particular transaction was shown at a higher amount than that authorised on the speedpoint machine and what was shown on the speedpoint slip. I queried this via email with FNB and sent a copy of the speedpoint slip as proof. After much chasing for an explanation I have it in writing from FNB that the merchant cannot subsequently alter a speedpoint transaction but despite this having been "escalated" within FNB I have not received any clear explanation from them as to how this could occur. So I suggest you check your bank accounts carefully.