"He who increases his wealth by excessive interest gathers it for one who has pity on the poor" (28:8). How does that happen?

The verse doesn't say how, or when. It says only where the wealth ends up. Excessive interest is two Hebrew words, neshekh and tarbit, and the law has the same pair, of "your brother" who "has become poor" (Leviticus 25:35): "Take no interest from him or profit" (Leviticus 25:36). Tarbit comes six times in the Hebrew Bible, and every time neshekh is in the same verse. Ezekiel has the pair four times, as in "he who hasn’t lent to them with interest, hasn’t taken any increase from them" (Ezekiel 18:8). In Exodus, too, the command about interest speaks of one who is poor: "If you lend money to any of my people with you who is poor, you shall not be to him as a creditor. You shall not charge him interest" (Exodus 22:25). Has pity on the poor in 28:8 has the two Hebrew words of "He who has pity on the poor lends to the LORD" (Proverbs 19:17), and the notes on Proverbs 19 look at them. Another saying ends in the same kind of way: "the wealth of the sinner is stored for the righteous" (Proverbs 13:22). These are sayings. 28:8 says nothing of how the wealth passes from one to the other, and these notes apply it to no one's money or lending today.