[Footnote 3: _De Cont._, ii. 11.] [Footnote 4: _Op. cit._, IV. xv. 10.] The price fixed by common estimation was therefore the one to be observed in most cases, and it was at all times a safe guide to follow. If, however, the parties either knew or had good reason to believe that the common estimation had fixed the price wrongly, they were not bound to follow it, but should arrive at a just price themselves, having regard to the various considerations given above.[1] [Footnote 1: Nider, _De Cont. Merc._ ii.: 'Si vero scit vel credit communitatem errare in estimatione pretii rei; tunc nullo modo debet eam sequi; quia etiam si reciperet verum et justum pretium, tamen faceret contra conscientiam.'] It did not make any difference whether the price was paid immediately or at some future date. To increase the price in return for the giving of credit was not allowed, as it was deemed usurious--as indeed it was. It was held that the seller, in not taking his money immediately, was simply making a loan of that amount to the buyer, and that to receive anything more than the sum lent would be usury. Aquinas is quite clear on this point. 'If a man wish to sell his goods at a higher price than that which is just, so that he may wait for the buyer to pay, it is manifestly a case of usury; because this waiting for the payment of the price has the character of a loan, so that whatever he demands beyond the just price in consideration of this delay, is like a price for a loan, which pertains to usury. In like manner, if a buyer wishes to buy goods at a lower price than what is just, for the reason that he pays for the goods before they can be delivered, it is likewise a sin of usury; because again this anticipated payment of money has the character of a loan, the price of which is the rebate on the just price of the goods sold. On the other hand, if a man wishes to allow a rebate on the just price in order that he may have his money sooner, he is not guilty of the sin of usury.'[1] If, however, the seller, by giving credit, suffered any damage, he was entitled to be recompensed; this, as we shall see, was an ordinary feature of usury law. It could not be said that the price was raised. The price remained the same; but the seller was entitled to something further than the price by way of damages.[2] It was by the application of this principle that a seller was justified in demanding more than the current price for an article which possessed some individual or sentimental value for him. 'In such a case the just price will depend not only on the thing sold, but on the loss which the sale brings on the seller.... No man should sell what is not his, though he may charge for the loss he suffers.'[3] On the other hand, it was strictly forbidden to raise the price on account of the individual need of the buyer.[4] [Footnote 1: II. ii. 78, 2, ad. 7. See _Decret. Greg._, v. 19, _de usuris_, cc. 6 and 10.]