What did Martin Luther King, Jr mean when he used banking metaphors in his speech at the March on Washington?

In his
speech at the March on Washington, best known for his "dream," King described the
United States as having "defaulted" on a "promissory note insofar as her citizens
of color are concerned." By this King meant that the United States was founded on the
promise that all men were created equal, and that everyone in the nation, including
African-Americans, was entitled to equality under the law and equality of opportunity. This
"promissory note" was first issued in the Declaration of Independence and reiterated
in the Thirteenth, Fourteenth, and Fifteenth Amendments. But it was one that was clearly not
being honored, as Jim Crow laws, discrimination in voting, and de facto segregation in American
cities effectively made African-Americans second-class citizens. So when King said at the March
that "we have come to...cash a check" he meant that the purpose of the Civil Rights
Movement was to force the federal government, and American society as a whole, to live up to its
promise of equality for all Americans. It was a powerful rhetorical turn, because it emphasized
that for King, the Civil Rights Movement was not about asking for charity. It was about claiming
things that all Americans were entitled to, or forcing (not requesting) the United States to
honor its obligations. In the context of the march, this meant passing federal civil rights
legislation.