*****ing from SA Citizens

This is one of the Banks that's *****ing from Citizens. I made a purchase online using my FNB App,I used my own available funds as per screenshots I sent to them,Later in the month they changed the sequence of transactions and make it look like when I make a purchase I had no money in my account. They then charged me fee excess item of R155 and claimed they honour the transaction on my behalf. How can they honour what has been honoured already,I don't have an overdraft facility with them but they still mess up account.