I think the word "troll" was raised as most people would not condone the resulting action - thus causing a debate and inevitable "outrage" at the action.
I'm 50/50. I don't agree with keeping it, I see it as partly theft (and partly the sellers error). It's like finding a wallet loaded with £50 notes outside a police station.
Yes, I've made mistakes, but trust me, I regret every single one. So yeah, I'd hand it in and post that back. That's my opinion. I don't agree with yours but most people were not going to condone this
thread and pat you on the back, so to speak.
No-one can say "hand it back" - it's not for anyone to tell you what to do.