The MSC is probably not going to do anything for you. Take your receipt for the return and go back to the store where you bought and returned it and 'give em Hell'. Be clear about what you want and take it to them in written form:
1) For them to correct their records to show that you owe them nothing and immediately give you a signed statement in writing to that effect.
2) Get the collection agencies to stop calling for a non-existent debt.
3) Notify the credit bureau(s) that the negative report was their error and should be purged.
Give them 30 days to present you with proof that these actions have been completed or you will notify your state's Consumer Affairs (whatever they call it in your state) and begin legal action.
At no point in these conversations would I mention that this had been a mystery shop.
Edited to add: Take them copies of your paperwork and safely store your originals.
Edited 1 time(s). Last edit at 08/20/2020 06:52PM by Flash.