Stealing?

I thought there was only one way to define stealing.  Taking something that doesn't belong to you.  A friend views it differently.  The cashier yesterday gave me to much change.  When I returned it, my friend looked at me as if I had lost my mind.  They asked me why I gave the money back.  I told them I couldn't keep it because it was stealing.  They said it was the cashier's mistake and I should have kept it.  When I talked to other friends and family members I got mixed reactions.  Some said I should have kept it and others said I did the right thing.  How we define stealing has a lot to do with how we are raised, I guess.  I was surprised that my friend told me to keep the money because they had a strict Baptist up-bringing.  I thought they would have the same view about it as I did.  I was not raised in a home with religion but my parents had firm views about what they believed was right and wrong.  What I call stealing my friend believes it is just profiting off of someone elses' mistake.