In the single line context (unclear what the rest of the passage includes), it suggests the frame of mind of the narrator, considering the afterlife, may be juxtaposing and contrasting part of the religious significance of money versus values e.g. "Render under Caesar that which belongs to Caesar and render under God that which belongs to God"; or calling to mind other expressions relating to money and the afterlife like "you can't take it with you" versus "whoever has the most money when they die wins", or even the fare to cross the river Styx.
Also, in the context of 1970s New York City, a corporate chain store might break a twenty if you asked. A small shop or street vendor would be more likely to require that you buy something; and at that, may just not have change for a twenty in any case.