Standard Bank ruining my credit profile!!!

I had an account with Standard Bank which had been handed over to a Third party debt collector, this was because I had been unemployed for a period and not been able to manage to pay my account. As soon as I found employment I settled the account in full and it was closed, I even confirmed with Transunion that the account had been closed. The Third party debt collector had given me a discount on the balance and once I paid the amount I sent proof of payment and the account was closed.

To my surprise now 3month after the account had been closed Standard bank has now referred my account to another Debt collection company and are now demanding the amount that had been discounted??? What ********* behaviour is that?? This has now harmed my credit scoring and I am disgusted by the unproffessionalism from Standard Bank.