The Missing Dollar-Brain teaser

Three friends check into a hotel room that costs $30. They each contribute $10 and pay the bill. Later, the hotel realizes they made a mistake and the room only costs $25. The bellboy is sent to return $5 to the friends. Unsure how to split the $5, the bellboy decides to give each friend $1 and keep $2 as a tip. Now, each friend has paid $9 (totaling $27), and the bellboy kept $2, which sums up to $29. Where is the missing dollar?