Do I trust them to return it to the correct person etc..
Either way, I am sure that whoever it is is nervous about their money and like this I can return it to them tonight..
Great idea, tizke lemitzvos.
I’d contact the bank the time and amount, and leave your phone number, they could choose to share your number with the potential loser to call you, or not.
Once it’s returned, I wonder what “a large sum of money” is. My seven year old found $100 outside a local shop a few months ago, I left my number inside and nobody claimed it. My son tried asking me why I’m leaving my number, don’t I want him to keep the $100? It was an opportunity for a teachable moment, I told him that if someone claims the $100 I’ll give him a different $100, since the Mitzvah is infinitely more valuable.