Laesio enormis

Laesio enormis, Latin for abnormal harm, is a legal doctrine that gives a contracting party the ability to rescind an agreement if the price of the exchange is less than a certain proportion of its actual value, for instance one half or two thirds. The principle was developed as a way to ensure that people received a just price, iustum pretium, in an exchange. It stood in opposition to the Imperial Roman view, found in the Corpus Juris Civilis, that the parties to an exchange were entitled to try to outwit one another. The source article is brief and does not say which jurisdictions apply the rule today or what fraction each uses, so those specifics remain for later depth work.