(a) If the value was unknown on account of substantial error (e.g., a woman buys paste ornaments, thinking they are genuine diamonds), the contract is invalid; if it was unknown on account of the individual error of a contractant (e.g., a woman buys a diamond of great value for a few dollars, because the seller did not know the value of the jewel), the contract is unjust; if it was unknown on account of public error reflected in the current price (e.g., an art dealer buys at a low price a masterly painting, because his superior judgment enables him to recognize in it qualities which others did not perceive), the contract is both valid and just. It is also lawful to buy at the present prices when one knows from sources that one can honorably use that the objects purchased will soon rise greatly in value, for one is not bound to share with others one’s personal knowledge.
(b) If the hidden value is no man’s property or is abandoned (e.g., a man buys a field in which he, but not the owner, knows that a treasure is concealed, or he buys a goose and finds gold pieces in its stomach and cannot discover how they got there), the buyer is entitled to acquisition; but if it has an owner who can be discovered (e.g., a man buys a coat in a second-hand store and discovers a large quantity of money in bills sewed inside the lining, and is able to trace back the coat to its former wearer, if he tries), the buyer is bound to restitution.
2130. Obligation of Restitution on Account of Unjust Prices.—(a) Unjust Possession.—If there was bad faith without fraud, the seller should restore the difference between what he received and the highest current price, the buyer the difference between what he paid and the lowest current price. Overcharges or underpayments in conventional prices should be compensated for according to a reasonable standard, such as the decision of the experts.
(b) Unjust Damage.—If there was bad faith and fraud with resultant damages to one of the parties, the losses should be made good, even though the just price itself was not violated by excess or defect (see 1762).
(c) Nullity.—If there was good faith on both sides, there is no obligation of restoration, unless we suppose substantial error, lack of proper consent, conditional agreement, etc., which make the sale null or rescindable (see 1725).
2131. Injustice Regarding the Thing Sold.—Having spoken of the injustices committed in reference to the price, we shall now treat of the injustices committed in reference to the thing sold. The contract supposes that the thing sold be of a certain character, and hence injustice is done if one of the parties wilfully misleads the other about that character.
(a) Thus, the species of the thing sold enters into the contract, and so it is unjust to deceive another person about the nature of the thing that is being sold (e.g., if the seller gives inferior substitutes or adulterated goods to those who desire the genuine and pure article, or if the buyer deceives an inexperienced merchant into thinking that the high-grade clothing material he has for sale is low grade).
(b) The quantity of the thing sold is also a part of the contract, and it is unjust to take advantage by giving less or taking more than is agreed on: “Thou shalt not have divers weights in thy bag, a greater and a less, neither shall there be in thy house a greater bushel and a less” (Deut., xxv. 13, 14).
(c) The quality of the thing sold belongs to the contract, and hence there is fraud if one of the parties deceives the other about it (e.g., if the horse sold is sickly or slow, when he is supposed to be healthy and speedy).
2132. Defects in the Thing Sold.—If there are defects in an article sold, but a fair reduction in the price is made on account of the imperfection of the article, there is no injustice in the price. But the seller is unjust, nevertheless, if he conceals the defects in spite of a contrary condition in the contract, for he injures the buyer by leading him into an agreement against his will.