If the $1 bill was marked in any way (eg. http://en.wikipedia.org/wiki/Wheres_George%3F http://en.wikipedia.org/wiki/Wheres_George%3F), SDF might have some clue

If the $1 bill was marked in any way (eg. http://en.wikipedia.org/wiki/Wheres_George%3F http://en.wikipedia.org/wiki/Wheres_George%3F), SDF might have some clues. If the post office keeps any logs of mail (eg. letter posted from box on X street to Y address) then that might also provide some useful information. Particularly so if the postbox had any security cameras watching it. Of course, these approaches require a few organizations to be working cooperatively to find voidnull.