Abstract
Is a sales contract valid regardless of the agreed price ? Should the parties be allowed to deceive each other ? Two fragments often read as " liberals " assert that it is natural in sale and hire to " se circonvenir " about the price (D. 4.4.16.4 Ulpian, book XI On the edict; D. 19.2.22.3 Paul, book XXXIV On the edict). But this " circonvention " is aimed at the ordinary dynamics of negotiation (overbids/returns), as distinct from fraud. When fraud alters equivalence and vitiates the consent of one of the parties to the contract - for example, falsified weights in a sale of oil -, the injured party obtains rebalancing (D. 19.1.32 Ulpian, book XI On edict). This is the line of the praetor's edict against fraudulent behavior (D. 4.3.1 pr. Ulpian, book XI On the edict).
The decisive text is a rescript from Diocletian : in the absence of metus or dolus, the mere fact of having sold " a little below " is not sufficient to rescind, as the sale is formed after a negotiation in which each party naturally tends to his own advantage. However, if less than half of the iustum pretium (at the time of sale) has been paid, rescission is open(Code of Justinian, 4.44.8). This " lesion of more than half " is not a late moral exception, but the legal application of the principle of commutative justice : beyond a certain difference, the exchange ceases to be equivalent and the contract loses its function. The one-half threshold thus constitutes an objective criterion, in line with the case - much earlier than Diocletian's rescript - of the house already burnt down at the time of sale. Nerva, Sabinus and Cassius considered the sale null and void(nihil venisse), but Neratius qualified : if more than half the house had burnt down, the buyer was not bound ; if half or less had burnt down, the sale was maintained with a reduction in the price set arbitratu viri boni (D. 18.1.57 pr. Paul, book V On Plautius). Since classical times, the criterion of one-half has thus appeared as a threshold of equilibrium. Transposed into modern law, the idea reappears in art. 1674 C. civ. (lésion de " plus de sept douzièmes ").
In short, Roman law protects contractual freedom (negotiation and the risk of " bad deal ") while drawing two red lines : the integrity of consent (absence of threat and fraud) ; the minimum equivalence of the exchange, guaranteed by the threshold of half the iustum pretium (Diocletian's rescript), a threshold already foreshadowed by the solution of Neratius on the sale of the burnt-down house.
