FNB Share Investing buys more than instructed and expects you to pay extra

I instructred FNB to buy Ashburton1200 shares every month with what ever cash is availabe in my investing account. They went ahead and bought R1500 more worth of shares than what was available in cash, and then sent me an email (addressed to someone else, which they recalled and rectified with my name later) half way through the month informing me that it is now my responsibility to pay the outstanding amount.

I phoned the call centre and the explanation given is that the price of the share changed from when the instruction was given to when the actual transaction took place.

I feel that this is a problem with their system and that it should do the neccesary recalculation, and not expect the customer to pay in extra over and above the maintance amount which he/she allocated in their budget for saving.

I will not be using this service/broker anymore, especially since there are much cheaper and easier alternatives out there.