VANISHED R500

on the 25th February I mistakenly deposited R500 into my son's (***) Loan Account. My son had paid off his loan by the time the I made the deposit. The BANK has yet to REFUND me the money,they keep sending me from pillar to post, Can someone assist in getting my money back, we have sent all required documents, my bank statement from my bank reflecting the deposit, still dololo.