One of the indisputable legal-historical rules in the Roman legal system is the rule regarding price reduction, which is similar to the institution of arsh (the difference in financial value between a faultless and defective property paid to the customer) in the Islamic legal system. In this paper, an attempt has been made, by an analytical method, to elucidate both ancient legal rules by redefining them, in particular with regard to the general theory of arsh, and to conclude that both have a common goal, which is to compensate the injured party with the aim of preserving the contract. Moreover, by redefining the opinions of the great Islamic jurists, an attempt has been made to expand the definition of defect and conditions regarding the application of arsh so that by preserving the traditional Islamic structures, the law of compensation in Imami jurisprudence and Iranian law shall be in line with that of advanced western laws. The abovementioned remedy is devoted to defect, since in Imami jurisprudence and Iranian law the description of faultless property is more important than other features, and this has caused a difference in the effects. Therefore, in this article, we shall prove that in Imami jurisprudence and Iranian law, if in bilateral contracts the descriptions of goods delivered are not in conformity with that required by the contract, the party seeking compensation may receive an amount equal to what is not in conformity with the contract, exactly as we have in the price reduction rule.