Missing Payment - Money floating around somewhere

I'll start off by saying that the mistake was initially created by myself, but the response and "LACK OF HELP" received from FNB leaves much to be desired.

I mistakenly gave out the wrong account number (account of mine that was closed last year September) to a customer of mine. He is also an FNB customer and paid money (missing money) into this closed account of mine from his FNB account. This was 6 days ago today. (Payment was made 15 March 2018, today is 21 March 2018)

I would assume that since this was from an FNB customer to another FNB customer that this transaction, however wrong it was, stays an internal transaction, that should be able to be tracked somehow?

Myself and the Client (Whom has been very calm and helpfull regarding the incident) has contacted FNB online banking, FNB general enquiries and just about any contact number available to find out where the money is and how to get it back. We were initially told that, seeing as it was from FNB to FNB it would take 48 hours to bounce back into he's account. WE WAITED. Day three came with no money so we both phoned FNB online banking again. We got no clear answer. He was told that I need to go into the bank with my ID so they could check the closed account that was in my name to trace whether the money was there. I reluctantly took of work to go check my none existing account, and went into the bank (FNB The Grove Mall - Pretoria) expecting to walk out with at-least some idea of where the money was. SORRY WE CANT TRACK IT ON YOUR SIDE, MY CUSTOMER(PAYEE) NEEDS TO GO INTO THE BANK. What? After all the trouble we went through I was not going to tell my customer to go into the bank. I phoned several numbers and got a few new answers, none were really helpful. The one I chose to go with was that the money could take up to five days to be paid back to he's account.

So lets sit and wait and hope for the best.

Here we are today. No money, no answers and not one step closer to getting the problem sorted out. So, tomorrow is a new day, and hopefully someone who has gone through the same ordeal can shine some light on the mishaps.