In the grand scheme of things, you don't have to do anything.
But, if you want to stay on the good side of the law, you might want to think twice before squandering that briefcase full of cash you found. That’s because most states have laws pertaining to when the playground rule of finder's keepers applies.
Generally, if you find more than a few bucks, not doing the right thing could be criminal, after all cash is not likely to ever be "abandoned" property (which is usually completely legal to take). Though, as one Michigan man is learning, there may not be much police can do when a box full of cash falls off the back of your truck and goes blowing in the wind. Only about $7,000 of his $30,000 in cash has been recovered.
Under several states' laws, if you find more than a certain amount of money, you are required to take it to the police if you can't identify the owner and return it yourself. The amount of money that requires you to do so varies by state. For example, in New York, it is $20, while in California it is $100. Typically, the laws will honor finder’s keepers. If after turning in cash or lost property to the police and the true owner cannot be found, after a period of time, the finder may become the keeper. In some places however, depending on the value of the item, there may be certain restrictions.
Finder or Stealer
While a finder of cash may think that no one will ever know they found what they found, when large sums of money, or important property, go missing, people tend to notice and look for them. Just ask those guys who found that prototype iPhone a few years back.
If you find lost property that is valuable or clearly not abandoned, keeping it could result in you facing theft charges. So like the rule of the playground requiring you to loudly announce finder's keepers, if you find lost property (including cash) in the real world, you need to let the authorities know, especially if you want to legally keep it.