Incorrect settlement amount paid

I had a claim with King Price for an accident on 25/08/2015 and requested the settlement quote amount from my bank. This was sent to KP on 31/08/2015. The letter stated that the quote was only valid until 04/09/2015. On 04/09/2015 Lizette indicated that she would request a new settlement letter from the bank because they, KP, were taking to long and wouldn't be able to settle by the 4th. She never bothered to do this and simply took a guess as to the amount owing to the bank and paid according to the old settlement letter on 11/09/2015, 5 working days or exactly a week, after the letter had already expired. On the same day I received the balance of the claim amount into my account. I assumed that they had done what they had said and done their job correctly and went ahead and paid this amount out as a deposit on another vehicle. This weekend my bank took another installment from my account. I asked Lizette to send me the 'new' settlement letter she had requested and got a very arrogant reply saying that she hadn't done this and paid on the old settlement letter. ABSA has now taken money from me to cover interest on this account because of the late payment. Why should I be liable?