What is "laesio enormis"

Laesio enormis (Latin: abnormal harm) is a legal doctrine that gives the ability of a contracting party to rescind an agreement if the price of exchange is less than a certain sum (for instance one half, or two thirds) of its actual value. The principle was developed as a way to ensure that people received a just price in exchange, and 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.