It is important to bear in mind that the source domain of the metaphor, the domain of financial transaction, itself has a morality: It is moral to pay your debts and immoral not to. When moral action is understood metaphorically in terms of financial transaction, financial morality is carried over to morality in general:

Real people have embodied minds whose conceptual systems arise from, are shaped by, and are given meaning through living human bodies. ... In asking philosophical questions, we use a reason shaped by the body, a cognitive unconscious to which we have no direct access, and metaphorical thought of which we are largely unaware.

Reason is not purely literal, but largely metaphorical and imaginative. Reason is not dispassionate, but emotionally engaged. ... Rather, the mind is inherently embodied, reason is shaped by the body, and since most thought is unconscious, the mind cannot be known simply by self-reflection.