This is a no-brainer for me. I'd stop and grab the money bag, then I'd catch up to the truck and flag them down. Then I would wag my finger in their faces and call them every name in the book for allowing MY MONEY BAG to fall out of the truck. Those armored car dudes are always so careless with my money.

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This is a difficult question to answer. I like to think I'd do the right thing and return it...no matter the amount. But if I were in desperate need of the money, well I just don't know what I'd do. I do know one thing, taking someone's money and giving it to the poor is not my right and IMO doesn't make it any less stealing. I don't believe in socialism. I am by no means a wealthy person (not even close), but there is truly little that I want and little that I need. I'm already comfortable with what I have. And when I look at myself in the mirror every morning I like knowing I've always tried to do the right thing (although I don't always succeed).
Nope, believe it or not you skeptics, but I would make every effort to return the ENTIRE amount of money and I'd feel good for doing it because it is
not the same as finding coins in the playground. The coins in the playground don't have BRINKS printed on them.
I remember a few years ago when an armored vehicle lost bags of coins on a street in Florida (I think it was Florida). The news crews videotaped dozens of people grabbing the money and running away with it. It was appalling and disgusting. They knew the money belonged to the armored car company, but they didn't give a dang about that. All that those people cared about was "getting something for nothing."
And "sticking it to the man" (apparently the insurance man, which is in reality all the people who pay the premiums...which equates to YOU and ME) in some perverted sense of the Robin Hood mentality. Robin Hood was a crook and to emulate him is to be a crook, too.
Scoff if you must, but I'd return the money. If I found an unmarked stash under a rock in the National Forest, I'd keep it and pray the dopers didn't track me down and kill me to get their ill-gotten gains back.