  The Consumer Financial Protection Bureau defines the notion of payoff amount
as the amount that has to be payed at a particular time in order to completely
pay off the debt, in case the lender intends to pay off the loan early, way
before the last installment is due (CFPB 2020). This amount is well-understood
for loans at compound interest, but much less so when simple interest is used.
  Recently, Aretusi and Mari (2018) have proposed a formula for the payoff
amount for loans at simple interest. We assume that the payoff amounts are
established contractually at time zero, whence the requirement that no
arbitrage may arise this way
  The first goal of this paper is to study this new formula and derive it
within a model of a loan market in which loans are bought and sold at simple
interest, interest rates change over time, and no arbitrage opportunities
exist.
  The second goal is to show that this formula exhibits a behaviour rather
different from the one which occurs when compound interest is used. Indeed, we
show that, if the installments are constant and if the interest rate is greater
than a critical value (which depends on the number of installments), then the
sequence of the payoff amounts is increasing before a certain critical time,
and will start decreasing only thereafter. We also show that the critical value
is decreasing as a function of the number of installments. For two
installments, the critical value is equal to the golden section.
  The third goal is to introduce a more efficient polynomial notation, which
encodes a basic tenet of the subject: Each amount of money is embedded in a
time position (to wit: The time when it is due). The model of a loan market we
propose is naturally linked to this new notation.
