Latin merces (wages, price, reward) shifted in Christian Latin to reward from God, then misericordia's cousin merci (French, thanks, mercy). English mercy took the compassion sense; merchant, merchandise, market (mercatus), mercenary, and commerce kept the cash. Wednesday's French name, mercredi, still pays Mercury, the merchants' god.
The theology embedded in the word: mercy as unearned payment, wages you did not work for, which is precisely how Pauline grace was preached in Latin Europe. The marketplace metaphor is not a corruption of the doctrine; it was the doctrine's first language.
Mercurius presides over merchants, messengers, thieves, and crossroads, the god of every transaction, honest or otherwise. That mercy's root answers to his portfolio suggests the ancients saw no contradiction: every exchange, even of forgiveness, happens at his crossroads.
What is forgiveness if not a price quietly declined?