What is a really hard, IMPOSSIBLE to solve brain-teaser?

Three men are travelling and get tired, so they decide to spend the night at a nearby inn. When they go to the front desk, the innkeeper charges them $30, ($10 each), and the three men go into their rooms and go to sleep. Then the innkeeper realizes that he was only supposed to charge the men $25 total, so he gives $5 to the bellboy to give to the men. But on the way to the men's rooms, the bellboy thinks to himself, "Hey, I've been so good, I deserve some money." So he keeps $2 and gives the remaining $3 to the men. Because they each get $1 back, they paid $9 instead of $10 each. But 3 x 9 = 27. But $30 - the taken $2 = $28. Where is the missing dollar?