When concluding a contract for a number of emerging transactions, such as purchasing in advance and goods production orders, the price of the goods is not clear and definite. While these contracts are not acceptable based on the Iranian legal system according to articles such as 190 and 338 of Civil Law. Therefore, this study with a descriptive-analytical approach has compared the nature of the condition of thaman (the amount paid by the buyer and received by the seller in commutative transactions such as sale and service) determination in the legal system of Iran and Egypt. In both legal systems, the need to determination of thaman is specified. In the Egyptian legal system, the capability of knowing thaman is one of the examples of its determination, but the criterion in the Iranian legal system is the realization of detailed knowledge of the thaman, except in indulgence contracts, such as a contracts of guarantee, gift, peace, etc., in which concise knowledge is sufficient. The present study, after examining the legal thoughts of Iran and its jurisprudential documents, has not accepted the necessity of thaman determination as an existence condition. The jurists have interpreted the meaning of the mentioned condition as non-existence, that is, the thaman of the transaction is not risky. However, the acceptance of the Egyptian legal system, that is, the capability of thaman determination is correct and legal when it is not risky. For this reason, the Iranian civil law in determining the thaman, such as Article 216 of Civil Law, is considered contrary to the above jurisprudential interpretation.