Sorry, but you had absolutely no entitlement to the money. There's been instances in the past where banks have misdeposited large sums of money into the wrong account. The bank's mistake doesn't suddenly make it the account holder's money. In fact, withdrawing the money could be illegal, depending on the jurisdiction. And the bank can and will correct the misdeposit and can even withdraw interest, if any, that may have been earned on the deposit while it was in the incorrect account.
In the end, it all boils down to a question of integrity. In this case, the MSC caught their mistake. If they hadn't, would you have mentioned it to the MSC once you noticed the issue? I recently had an MSC overpay me $30 (I did two shops at $15 each, but they paid me at $30 each). I immediately notified the MSC who, in the end, just told me to keep it. I could have said nothing, but if they uncover the issue later and I haven't said anything, I could lose my standing with the company if they think I've been dishonest.
Shopping central Arizona.