The story is set in a no-name kingdom, seemingly medieval but with certain modernisms. The cambist of the title is a minor worker, whose daily routine is interrupted by Lord Iron, who has come to play a cruel prank on him, as he was known to do. Lord Iron demands that the cambist make an exchange for him, presenting him with the most obscure currency possible. The cambist daringly, almost impossibly, succeeds, and as he explains his reasoning, rather surprisingly earns Lord Iron's respect.

Later, Lord Iron comes up with a more demanding and seemingly impossible exchange request, one with consequences. The cambist again succeeds, and again his reasoning, despite its counterintuitiveness, carries the day. And then comes one more request.

The mathematical content is limited to the explanations of the chain of equalities that led the cambist to his conclusions. Curiously, this is actually the most interesting part of the story!

It was published in the unusual anthology Logorrhea (online here), a collection of stories, each one keyed to a winning word from the annual National Spelling Bee. Abraham's story was based on "cambist", the 1977 winning word. (A cambist is a banker, especially one who specializes in currency exchange.)

This is a work commonly cited as "rational fiction", probably for the ingenuity and calm the main character shows when faced with extremely powerful enemies.