3 friends are in town for a convention and decide to share the expense of a hotel room.

The clerk informs them that it will cost $30 for the night.
Each takes out $10 and pays the clerk.

When the hotel manager arrives he notes a discrepancy on the books, the room only costs $25 per night, the clerk has overcharged them.

He gives the clerk $5 and tells him to go and pay them back.

The clerk doesn't know how he's going to split $5 evenly 3-ways so he gets a bright idea.

He tells the guests that there has been a mistake and they were overcharged. He returns each $1 and decides to pocket $2.

Each gave $10 and got $1 in return so each paid $9 for the room.

Question:
Since each paid $9 ($9 x 3 = $27) and the clerk kept $2 ($27 + $2 = $29), where is the extra dollar?