If there is cash on the street, you are permitted under Torah Biblical law to take it as long as the owner is not around to claim it and you have no way of knowing who the owner is.
If the owner has lost the money and has given up all realistic hope of recovering it, you are permitted to keep it. Because the money no longer has an owner.
Of course, if you find a wallet with someone's name and address, you are required to return it to the original owner.
All people give up realistic hope of recovering money because of thieves. That doesn't give one the right to steal the money because it's still his and he wouldn't want someone to gain because he has lost money. I don't know how the Torah could be interpreted like that because this is stealing. And whats the limit here? What if you find $10,000? And what if a cashier gives you too much change? Should you steal it because the cashier made a mistake?
All people give up realistic hope of recovering money because of thieves.
If you make a statement, in which you use "all" and reference it to people's opinions and attitudes, it invariably is likely to be wrong. And your statement is no exception to this rule, very many people if say they have a twenty dollar bill in their pocket and lose it in the street and only realize they lost it a couple of days later, will reckon that either it got swept up with the thrash on the street by the street cleaners and ended up in the town dump or somebody saw it and picked it up and the person who lost it, would very possibly reckon that, hopefully if somebody finds the money, they will be a good person and they will deserve to find it and use the money for a good purpose.
What if you find $10,000?
What if, you find $10,000 and you know it belongs to the Government Bank of the Third Reich and you are a Jew in the Warsaw Ghetto during World War II and you could use the money to buy food for starving people in the Ghetto and to buy rifles, do you hand over the money to the Nazis?
And what if a cashier gives you too much change?
If a cashier in a bank or a shop gives you too much change, you have to tell them if they give you one cent too much if the enterprise acts in a fair manner, if the bank or shop has knowingly robbed you before, I do not know about Jewish law on this point but in my view they should have been more careful counting out the change and hard luck to them, if they give too much change.
Best and Warm Regards
Adrian Wainer