Children

Somehow, he knows the precise moment
at which to tug the fishing line.

Past the unknowable edge
of every corner building,
he is a wiry boy with hair asunder.

His baby teeth are stalactites
lining the cavern of his soda-stained mouth.

He is waiting for you
to bend and pick up that dollar on the ground.

When you reach for it,
the dollar moves forward several inches.

Your cheeks turn red
as you notice the glances of the passers-by.

’Such wanton greed,’
you know they’re thinking.

A guffaw emanating from beyond the corner
attracts your murderous rage.

However, when you see the boy-
his tossed floppy hair strewn by urban windstrokes-
it is evident that your fellow pedestrians would scold you for such an act.

His resounding thigh-slaps are the fanfare
of a criminal who knows
his own invincibility.

Still, your inclination is to run him through with a large spear.

The last line makes me very, very unhappy. I really liked the poem until then...but that line is unnecessary and really ruins the feel of the whole poem.

But really, other than that I enjoyed this quite a bit. You did an excellent job at putting this into a specific time and place, giving the speaker surroundings, using a lot of description for this child.

Logic.

Somehow, he knows the precise moment
at which to tug the fishing line.

Past the unknowable edge
of every corner building,
he is a wiry boy with hair asunder.

This confused me at first because I didn't know that the fishing line was part of that old dollar bill trick. On that important first read I thought that he was actually fishing, and then...what? City? So...I would actually switch these two stanzas.
"Unknowable" is awkward. Find something else or leave it out entirely.

Grammar.

’Such wanton greed,’
you know they’re thinking.

This would be more correct if it was divided into two sentences:
'Such wanton greed.'
You know what they're thinking.
It still reads more or less the same way, but it's correct (and less confusing) this way.

Diction. For the most part, this is good, but there are some parts when the words used are out-of-place, usually because they're too "big" or formal.

A guffaw emanating from beyond the corner
attracts your murderous rage.

This uses a few words not normally heard in common speech: guffaw, emanating, attracts, murderous, and rage. Though the tone of this poem is not entirely conversational, it's also not an exercise in vocabulary.

However, when you see the boy

"However" seems a little too stiff and formal. You could just use "but."

Keep at it!
-Colleen