Worst experience trying to settle account

I am paid on the 27th of the month and in July i was paid early due to the 27th being on Sunday,for whatever reason absa did not get the July installment,was informed about the unsuccessful debit through sms which led me to call absa on the 28th of July requesting a settlement letter and paid the account within the validity period. Upon my surprise when im receiving a call from absa saying i am in arrears... the agent is saying the amount on the settlement letter excluded the installment for the current month continues to say i should have asked,it was my responsibility to say...Question is, How as a client would i know that the agent can choose to include or exclude certain amounts on a settlement letter as the agent i just spoke to said i should have asked "so the agent would dig deeper"? Why should i ask before they dig deeper, why not make the amount on the binding document is correct before issuing? What is the purpose of issueing a settlement letter if it means account wont be settled after? Why is Absa shifting blame on me when i dont know their processes and procedures? Now my credit record will be affected due to arrears, i have +R8000 in arrears when the installment was +R6900,Absa shifting liability on me when i did all i should have done to settle but they didn't

Please rectify this send the VAF +Natis documents for the car