What should you do with money you find?
Money you find does not belong to you. You hand it in at a lost property office or a similar place and stop thinking about keeping it. The owner can collect it there once he proves that it is his.
Whoever finds money is holding something that belongs to someone else. It is a trust. The first verse above says that trusts must be returned to those entitled to them. So the finder is not the owner but a trustee.
In the past, scholars laid down exactly how long a find had to be announced publicly. Some named a few years, others an even longer period, and only after that could the find be used. Such rules suited the circumstances of their time. Today it would often cost more to advertise a find for years than the find itself is worth.
So the basic principle is something simpler. A find is not personal property but a matter for the community. You take it to a lost property office or a similar place and then forget about it. Anyone who has lost something goes there, proves that it belongs to them, and gets it back. This is also the custom at large gatherings, where many people hand in things they find straight away.
People tell mocking stories of those who announced a find so quietly that nobody would come to claim it. Such tricks show how easily the wish to keep something can outwit the conscience. The second verse above warns against taking the property of others unjustly. It is best to be clear from the very start. This is not mine, and it will never be mine.