Six short lines are enough for Dave Bonta to stage a small disaster: someone expects payment and gets something foul. The money the speaker "was promised" turns out to be "some dark discharge / in a sack," and the speaker's anger lands on whoever urged them to "believe / in tomorrow." The central idea is disillusion. A promise about the future can end up worth less than nothing, because when tomorrow arrives it is actively unpleasant.
The title sets up the irony. A down payment is a first instalment, a gesture of good faith that says the rest will follow. Usually the buyer makes it, which allows a sharper reading: the speaker's belief in tomorrow was the down payment, handed over in trust, and what came back was the sack. The poem never says which reading is right, and the uncertainty suits a speaker who no longer knows who owes what.
The central metaphor is stated flatly. The poem does not say the money "feels like" or "turned into" anything. It simply "is" a discharge, as if the speaker were reporting a fact they have stopped arguing with. The vague "some" adds contempt, since the speaker can't be bothered to name the stuff. "Discharge" is a rich word here. It can mean a bodily emission, which fits the unpleasant "dark" and the bag-like "sack." In the language of money, though, to discharge a debt means to pay it off, so the word hints at a settlement that has gone horribly wrong. Both "discharge" and "sack" also belong to the vocabulary of losing a job, since a worker can be discharged or get the sack. The ordinary image of a money-sack full of coins is soured, and the speaker may be cheated, dismissed and disgusted all at once.
Then comes the turn: "o my thorn." This is an old-fashioned lyric apostrophe, the "o" of love poems and prayers, sitting oddly in a poem otherwise written in plain speech. The addressee is never identified. It could be a lover, an employer, a god, or hope itself. It could even be the speaker's own gullible self. The poem gives no way to decide, and it is stronger for that. A thorn is a small, sharp, lodged pain, the kind that keeps pricking, and it grows on the stem of something that looked like a reward. The possessive "my" borrows the grammar of tenderness, as in "o my love," and bends it into resentment. The speaker is still attached to the thing that hurt them.
The form carries the feeling. There is no rhyme, no capital letter, and no punctuation, so the poem has no full stop to close it. Counted by syllables, the lines run roughly seven, five, three, three, six, four. They shrink into the two blunt middle lines, "in a sack" and "o my thorn," where the heavy one-syllable nouns "sack" and "thorn" land like small blows. The hard sounds of "dark discharge" and "sack" add to the sourness. After that the lines lengthen again for the accusation. The break after "believe" matters too. For a moment the verb stands alone, as though the speaker had believed absolutely, before "in tomorrow" supplies the object and shrinks the belief to something flimsy. The word "believe" even has "lie" inside it, which may be an accident of spelling but fits the mood. The last word, "tomorrow," also has a soft, open ending that echoes "borrow" and "sorrow," so the future is bound up with debt and regret.
The poem is also framed by time. "Promised" opens it and "tomorrow" closes it, so both ends point forward, while the ugly present sits trapped in the middle. We never learn what was promised or by whom. That gap makes the poem easy to recognise from one's own life, because anyone who has been told to wait and trust will know the feeling.
The poem is significant for how much it does with so little. In six lines it combines financial and bodily language, a slangy voice and an old lyric gesture, tenderness and disgust. It ends on the word that caused the trouble and leaves the debt unsettled, so the reader is left, like the speaker, holding the sack.