Did GS actually say that he opened the envelope? Perhaps he only discovered that it contained a large sum of money after taking to someone inside the bank who opened it in his presence. If he left the bank with the envelope he still has the ability to use the account information from the enclosed deposit slip to deposit the money.
If there was so much money that the envelope would not enter the correct slot, a simple solution would b to go to someone in the bank and insist that the deposit be made at a teller's window inside the bank. I would not accept a promise of anyone in the bank to take care of it later.
Last year, in a large hardware store in Bahia Blanca I found a wallet (actually a "man bag"). I immediately took it to the customer service desk and showed it to the manager. It took about a minute for him to understand that it wasn't mine. He opened the bag in front of me. It was full of money, credit cards, DNI, driver's lisence, etc.. I asked him to use the public address system to call for the person who's name appeared on the DNI. I said that we only had to wait for the man with a fearful look on his face. In less than a minute a very nervous man approached the counter. I waved him over to us and smiled, telling him in Spanish that everything was there. He looked inside and thanked me. I told him he was welcome and walked away. I am not sure what the store manage would have done if I hadn't waited for the rightful owner to show up, but I had no intention of leaving until he did.