Appaling service delivery

I jave been struggling to change my personal loan debit order date. When I app****, I picked the 1st of the month for my debit order date. When the agreement came, FNB had changed it to the 25th. My salary has NEVER been paid on the 25th. I get paid on the 28th, sometimes later. When I queried FNB and asked for the date to be changed to the 1st of every month, they wanted a letter from my employer. No employer is going to state on record that some months they pay late. FNB has access to my account and can see for themselves. No reply to date to my request further. I explained that should the debit order go off on the 25th, I will encur extra costs as there won't be money in the account. This level of service delivery is appalling!