The speaker goes out walking with a worry they can't share, and the poem turns that worry into a single image: a new coin. The coin looks right. It has a good "stamp," a clean surface that suggests value. But in "the money," meaning in actual use, it's bad. So the trouble is something that seems fine from the outside yet doesn't hold up when it counts. The poem never says what the trouble is, which keeps it private, just as the opening line says.
The title, "Purchase," plays on this. A purchase can be a buying, but also a grip or hold on something. The speaker seems to have neither: nothing can be bought with this coin, and nothing can be firmly grasped.
The ending is bleak and quiet. The speaker eats dinner with the trouble, and it is "hard and cold," like metal. The mood is lonely, heavy, and restrained, with the worry sitting at the table like a companion.