If it was possibel to keep it without ever being caught, I would probably keep it. But if that is federal money, I'm pretty sure you would be found and prosecuted if you used any of the money and decided to keep it. This recently happend when someone found a bag of money that fell out of a armored car. The man decided to turn it in, but while they were investigating what happend to it, someone turned him in. They said they saw a male blah blah description, and turned him in. You would get caught really fast especially if it was a amount of money that big.
If you could keep it without ever getting caught, I sure would