Aristotle, Nicomachean Ethics
H. Rackham, Ed.

[9] For in buying and selling also this seems to be the practice¹; and in some countries the law does not allow actions for the enforcement of voluntary covenants,² on the ground that when you have trusted a man you ought to conclude the transaction as you began it. For it is thought fairer for the price to be fixed by the person who received credit than by the one who gave credit.³ For as a rule those who have a thing value it differently from those who want to get it. For one's own possessions and gifts always seem to one worth a great deal; but nevertheless the repayment is actually determined by the valuation of the recipient. But he ought no doubt to estimate the gift not at what it seems to him to be worth now that he has received it, but at the value he put on it before he received it.