This is very clever but I must admit I have seen it before...
It's all down to the way it is worded that causes the confusion BUT the key part is that the room cost $25-00
NOT $30-00
Therefor: -
Each Man Paid $9-00 so a total of $27-00...
So $25-00 for the room PLUS the $2-00 the Belboy kept = $27-00 = NO EXTRA DOLLAR