There are posters in a neighbourhood near mine that say, pretty much, “REWARD If you find a five dollar bill with one of the following serial numbers (ft32672 blah blah…)” and then there are pull off phone numbers at the bottom. At first, I guessed that it had something to do with a stolen wallet or something, but the handrwiting is pretty close to those “Make money working at home” signs. So, out of pure curiosity, what’s this guy playing at? Am i being too cynical, could it really be a plea for help?
Have you thought of calling the number?
From a pay phone if you’re afraid. 
Maybe it was just a fiver with special, sentimental meaning, or something.
Perhaps there’s a counterfeit bill… A common one that’s running rampant.
People call in, they have the bill. Cops answer. Where did you get it… To trace the counterfeiters.
Just a thought-zi-ball.
or…
It’s cheaper than cold calling. In other words it’s a common string and it your reward will be introduction to some marketing scheme.
or…
It’s the serials of an uncommon bill. Here in Australia each note has the signature of the current governor of the Reserve Bank and the current Treasurer (I think, too poor to check right now). So, if one of these good folk only serves for a short time that note (bill) is more valuable to a collector.
Give them a call, let us know.
If it’s a full serial number, it can’t be a common string unless it’s counterfeit.
Wouldn’t it be a lot quicker to just buy one from a collector? What are the chances that someone would bother to look at all their dollar bills and find one and call the person?
I agree.