A five-dollar dilemma

You walk into a bakery to meet a new acquaintance for coffee. You both order and while he leaves to use the restroom, you prepare to sit down at the table where you've both plopped your stuff. You glance down and see a wrinkled five-dollar bill on the floor directly beneath the table.

When he returns, you ask your companion (someone you've never met until now) if he dropped it. Nope. He didn't.

Now what do you do?
 
a) Leave it.
b) Take it.
c) Neither. If you choose "c" what do you do?

What are the variables that might cause you choose differently? Why?

a) If you were with someone you already knew well (a family member or a longtime friend)?

b) If it were a dollar bill? A twenty-dollar bill?
c) Other. What?



Image: betterthanblank.org