Incorrect Billing

Highly disappointed with services from woolworths loan department i called in to do a loan settlement, The lady (Darren) Incorrectly paid loan amount to incorrect loan, the outstanding loan did not get settled, she advised me that it would be corrected however when i received my statement it showed it was not corrected, Now i have been charged interest on a loan that should have been settled.I have also been billed the incorrect amount. Whose fault is this, its fine to make a mistake but when u dont fix it then it is unacceptable. Now im calling in again and i get told that its too late i will still be billed the incorrect amount. Pathetic service.