You always return the wallet. This is not a Christian thing this is just a human thing. But here’s the pros and cons.

Number one scenario: it belongs to an insanely rich person. If you keep this wallet they will barely miss that money because they have plenty, however the police will work very hard to find you and then they will beat the ever living shit out of you and you will be sent to jail for stealing from an oligarch.

Scenario two: it belongs to somebody for whom this is their entire life savings and without it they will become destitute. You will face no repercussions for keeping the wallet and the money because they have no power. But you will have completely destroyed someone else’s life which makes you a vile disgusting POS.