The speaker was promised money, and what arrives instead feels like something ugly: a "dark discharge" in a sack. The word "discharge" suggests waste or bodily fluid, so the payment, supposed to be a hopeful first step, has turned into something dirty and repellent. The title, "Down payment," makes the sting sharper, since a down payment is meant to be the start of something good, a sign that more is coming.
The poem's last lines turn from the money to a person, or perhaps a voice, addressed as "my thorn." This figure told the speaker to "believe in tomorrow," and the speaker now sounds bitter about it. A thorn is both a small, sharp pain and something that comes with a rose, so the address seems to mix irritation with a kind of grim affection. The poem is short and blunt, and its mood is rueful and betrayed. Trust in the future has been tested and found to cost something. The speaker's tone suggests tired disillusionment more than loud anger.