Refusal to change debit order date

I had taken a loan with FNB, they had requested various documents, to which I had sent. One of which was my latest payslip, which on the odd timing my salary was paid on the 24th of the month. The usual date my salary gets paid into my account is the 25th of every month. Getting my salary on the 24th of the month does not happen often, usually once/twice in a year. However, this particular month (July), was that odd one. I bank with FNB, all my accounts are at FNB because I thought they making banking easier. When I call to change my debit order date, they tell me I cannot because my last payslip reflected the 24th. I asked if i could send my previous payslips to prove this was just a once once case. I was told it is not possible, I had to send them proof, a letter from my employer. But is it not proof if I send my previous payslips. No, apparently not because the loan was based on the last payslip. So if I do not manage to change this in time, the debit order will be bounced back, with will affect my banking record and credit rating? FNB is frustrating and unhelpful, do not claim \How can we help you"when you cannot assist with a simply request of changing a debit order date!"""