Unable to pay an overdrawn account

In 2014 fnb was hit by a duplicate transaction processing system error which affect customers nationwide, by having their accounts double deducted or credited with duplicate amounts on their accounts. I was one of the customers affected. But positively my salary payment was doubled, however I was not aware as the duplicate transaction did not report the deposit into my account unknowingly I used some of the funds and a day later fnb sent messages informing me of the transaction processed in error. I went to my branch in Rose Bank and told them I could not afford to pay back all that money at once so we arranged that I open a savings account for my day to day banking and work on a payment plan for my now overdrawn cheque account. I deposited the agreed amount into the account, later in the year 2015 I was no longer able to perform the inter account transfer and kept getting an error of invalid account. And no body can assist me with the account details to pay the outstanding amount as consultants keep advising my account is flagged. Now I cant pay the account as nobody at fnb seem to know where this account is handled. And I am stuck because I need to settle the account.