5. Two Other Conclusions or Rules

143. But when an exchanger does not receive immediately that for which he does the exchanging, but reception of this sort is deferred, the question is what is right or of law?

I reply: besides the aforesaid rules [nn.139-142] pertaining to the just and unjust in individual contracts for present time, I add here these two rules: the first is that the exchanger not exchange or sell time, because time is not his; second, that he not put himself in surety of making a gain, and him with whom he exchanges in surety of making a loss (I mean by ‘in surety of’, always or for the most part).

144. From these rules [n.143] are plain many particular cases; for example, let the feast of the Lord’s Nativity [December 25] be called a, and the feast of St. John the Baptist [June 24] be called b. A given exchanger hands over his property to another at a. Either he was going to sell it then, or was not but at b:

145. If so [sc. at a], either he determines the price then according as the time is running at a, and he does an act then of mercy, because he is supplying then the need of his neighbor (before he is bound to supply it), namely when waiting until b for payment of it. Or he determines a price greater than is just at a, and then he is a usurer, because he is, against the first rule, selling time [n.143]; this is proved from Gregory IX, Decretals V tit.19 ch.10, ‘On Usury’.

146. But if he was not going to sell now but at another time when it would seem that, according to the run of time, he could make more gain - either, therefore, he sets down a fixed price then, or he does not but leaves certitude about the price to depend on something in the future.

147. If in the first way [n.146], then if he lays down a price according to what the thing is now worth, there is no doubt but that he does a great mercy. If, however, he lays down a price greater than it is now worth, but not so immoderate a price but that at the time of payment the thing sold would likely sometimes be worth more sometimes less, he is, by reason of the doubt, excused, because he is doing nothing against the aforesaid rule [sc. the rule not to sell time, n.143]. This is proved from ibid. ch.19 [see n.139 fn. 26]. -And if objection be made against this from what is said there [ibid.: “provided that at the time of the contract he had not been about to sell them”], I reply: what is contained there is a useful warning, not a necessary precept.

148. But if he abandon determination of a price dependent on the future value of the thing, then either for the determinate time of the payment, or for another time when it is not as a rule customary for the thing to have a greater value than when he gave that thing of his - and then he does an act of mercy, for example: “I allow it to you at as great a price as it will be worth at b, or at some time before b,” although however the thing will be commonly accustomed to be dearer at b than at any preceding time.

149. But if he want the price to be determined for some indeterminate time, so that, in this way, he put himself for the most part in surety of gain and the other in loss, as for example ‘I want you to pay me as much for it as it will be worth at any time up to b when it sells more dearly’, it is usury, because he sets himself up or his party for the most part for gain, and him with whom he contracts for the most part for loss; and then he has for himself what happens for the most part, and against himself what happens for the least part.

150. And another injustice is there as well, because on some determinate day and not at some vague particular time must he set out his property for sale, and it might at that time happen that it would be sold less dear than in a day between a and b that was dearer; and consequently, in such a pact he makes himself certain of a gain further than human industry could attain.

151. These then are the stated rules about the just and unjust in any selling and lending for the time now or in the future - and saying this about household [‘economic’] exchange, which is when the exchanger intends to accept the thing for which he does the exchange, because he buys it not to trade it but to use it.