FNB Home Loan Dept calculating repayment incorrectly

FNB Home Loan Department - PLEASE EXPLAIN HOW YOUR IT SYSTEM WORKS

I withdrew a PORTION of my FNB home loan this month i.e. NOT the full amount of the loan. I was VERY suprised when I was debited as if I had withdrawn the FULL amount. When I phoned to query this, Zanele informed me that "this is just how the system works". She then sent me a form to fill in a request to reduce the monthly repayment amount. I must say I am VERY disappointed that the FNB Home Loan System is unable to process such a simple transaction correctly. Either that, or someone "had finger trouble" when capturing this transaction.