n to be
made, for instance if a man be deceived in regard to more than half
the amount of the just price of a thing [*Cod. IV, xliv, De Rescind.
Vend. 2, 8].

On the other hand the Divine law leaves nothing unpunished that is
contrary to virtue. Hence, according to the Divine law, it is
reckoned unlawful if the equality of justice be not observed in
buying and selling: and he who has received more than he ought must
make compensation to him that has suffered loss, if the loss be
considerable. I add this condition, because the just price of things
is not fixed with mathematical precision, but depends on a kind of
estimate, so that a slight addition or subtraction would not seem to
destroy the equality of justice.

Reply Obj. 2: As Augustine says "this jester, either by looking into
himself or by his experience of others, thought that all men are
inclined to wish to buy for a song and sell at a premium. But since
in reality this is wicked, it is in every man's power to acquire that
justice whereby he may resist and overcome this inclination." And
then he gives the example of a man who gave the just price for a book
to a man who through ignorance asked a low price for it. Hence it is
evident that this common desire is not from nature but from vice,
wherefore it is common to many who walk along the broad road of sin.

Reply Obj. 3: In commutative justice we consider chiefly real
equality. On the other hand, in friendship based on utility we
consider equality of usefulness, so that the recompense should depend
on the usefulness accruing, whereas in buying it should be equal to
the thing bought.
_______________________

SECOND ARTICLE [II-II, Q. 77, Art. 2]

Whether a Sale Is Rendered Unlawful Through a Fault in the Thing Sold?

Objection 1: It would seem that a sale is not rendered unjust and
unlawful through a fault in the thing sold. For less account should
be taken of the other parts of a thing than of what belongs to its
substance. Yet the sale of a thing does not seem to be rendered
unlawful through a fault in its substance: for instance, if a man
sell instead of the real metal, silver or gold produced by some
chemical process, which is adapted to all the human uses for which
silver and gold are necessary, for instance in the making of vessels
and the like. Much less therefore will it be an unlawful sale if the
thing be defective in other ways.

Obj. 2: Further, any fault in the thing, affecting the quantity,