Nostromo, who worships his reputation, is becoming trapped. Allegorically, the issue reduces to this: the People must pay this tribute in order to remain the People. To refuse is to violate their social role and stand apart from "the crowd"; i.e., it is to lose their identity.
Note that the color of his mare is invoked, which consists of "silver"
(dreams) and "grey" (ambiguous ambition). Nostromo's conflicting dreams
are almost at the breaking point here.