FNB payment agreements mean nothing to them....they will take what they want when they want

Last week I called FNB Credit Card deviation to make an payment arrangement on my credit card... I received a call back to confirm that i understand the agreement and i received an sms the remind me of the payment arrangement. I just received another sms that notify me that the payment was deducted however the amount is more then the agreed upon amount? I call the bank now again and spoke to Larry Nckenna who then told me that he can see the arrangement that i have made but they would refund me the different because they can take whatever amount they want if there is fund available in my account? Please can someone explain to me why I then have an payment agreement with FNB if they just gone take what they wont out of my banking account??? PLEASE!! I asked to talk to a manager and was told I have to call back later because the team leader was busy......