finders keepers (losers weepers)
proverb

- used especially in children's speech to say that a person can keep what he or she has found and does not need to give it back to the person who has lost it (all the person who lost it can do is cry about it)

Merriam-Webster

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ORIGIN

This saying can be traced back to an ancient Roman law of similar meaning and has been expressed in various ways over the centuries.

Of particular difficulty is how best to define when exactly something is unowned or abandoned, which can lead to legal or ethical disputes.

One of the most common uses of "Finders, Keepers" involves shipwrecks. Under international maritime law, for shipwrecks of a certain age, the original owner may have lost all claim to the cargo. Anyone who finds the wreck can then file a salvage claim on it and place a lien on the vessel (Schiffspfandrecht), and subsequently mount a salvage operation.

In the United States, the Homestead Act allowed people to claim land as their own as long as it was originally unowned and the property was then developed by the claimant.

SYNONYM

One man's joy is another man's sorrow = Des einen Freud, des andern Leid