Finder finds wallet.
Finder takes money and spends it.
Finder hands in wallet to police.
Police hand wallet to Owner.
Finder discovers the next day that their friend is Owner and Owner is crying their heart out about how they are being evicted from their flat because they lost $200 in their wallet and got their wallet returned but the money had been stolen and now they can't pay their rent.
You find it, good for you. But that doesn't make you any less a common thief if you spend the money.