troll economics?

3 guys go out to eat and order a steak. The steaks cost $30 total so each one gave the waitress $10. The waitress was walking to the register and she realized that she had a $5 coupon. So she got $5 singles out of the drawer and gave each guy back one dollar and she kept $2 for herself. The guys then paid $9 each for the steak which is $27 plus the $2 the waitress kept is $29. What happened to the last dollar?

I put $20 in a box
You put $20 in the same box
We both know that there is $40 in the box
...I sell the box to you for $30
We both walk away with $10 profit.
Discuss :)