I totally disagree with some of the advice on here.
If I found $50 and there was no apparent way to return the money to the owner, I would thank the Lord and say a quick prayer for the person who lost it.
I believe sometimes God blesses people who are in need by letting them find money. How many broke mothers have been praying about what they would feed their child that night and found $20 on the ground.
Give it in to the ticket office? I would ask if anyone lost money, but hand $50 to a ticket agent?

Maybe I’m too skeptical.
One time I was in a drug store and I must have dropped $80 I had rolled up on my pocket. I was so angry because I’m sure it fell while I was on the line which means someone saw me drop it and took it. That is stealing.
But finding money lying in the street when you are not able to return it to the owner is not.
If you feel guilty, put it in the collection basket or something.