The pronoun us
refers back to the we
at the beginning of the sentence: those Jews who used their financial strength to buy back our Jewish brothers who have been sold to the nations.
Instead of now purchasing (and then freeing) brethren-made-slaves as prisoners of war, these richer Jews now need to spend their resources on purchasing brothers and sisters whom their fellow Jews have sold. Nehemiah makes plain how totally ridiculous the selfish actions of the nobles and the officials
are.
8 and said to them, “We, as far as we are able, have bought back our Jewish brothers who have been sold to the nations, but you even sell your brothers that they may be sold to us!” They were silent and could not find a word to say.