There really isn't a missing dollar. The riddle fools you into thinking that what the men paid plus what the bellhop received should equal $30.
The three men paid $27, the motel received $25, and the bellhop received $2. Total paid is $27 ($9+$9+$9) and the total received is $27 ($25+$2). The books balance and there is no missing dollar.
It is only a coincidence that what the men paid plus what the bellhop received comes close to the original $30 figure. Consider if the men had initially paid $60 ($20 each) for the room and the manager gave the bellhop $35 to return to the men. The bellhop could have given each man $10 back and kept $5. Each man has now paid $10 ($30 total) the bellhop has $5. $30+$5=$35 and we see that now we're so far off from the original $60 that you don't think to look for the "missing" $25.
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