Question

King uses words associated with money, such as "cash a check," "a promissory note," "defaulted," "bank ...bankrupt," Why do you think he does that?

Answer 1

king uses these banking words as metaphors. His training in music and experience in the history of civil rights helped him craft his masterpiece speech.
by using the words such as a promissory note, he implied the USA promise of freedoms, and that the USA had defaulted on this note. kings pointed out that this note had been issued in the declaration of independence.