Seyed Mostafa Khomeini was an able jurisprudent whose main characteristics were: critical study of the inferential viewpoints of other jurisprudents, precision, and raising issues and providing scholarly answers to them. One of his works, Kitab ul-Bai’ (Book of Transactions), is unique regarding the style and method of critical study of inferential decrees. Through his intellectual freedom, the Mostafa Khomeini studied Imam Khomeini’s viewpoints and either acknowledged or rejected them. Understanding the concept of transaction through its effects, giving originality to mu’atat in contracts, lack of application of the verse bai’ to the issue of mu’atat, reference to the tradition of saltanat regarding correctness of mu’atat, lack of domination of the rule of “No Duress” over rational verdicts and the selection of the highest price until the day of termination regarding guarantee and determination of the condition of wasted property are among his exclusive innovations regarding transaction. The present paper is an attempt to comparatively study the viewpoints of other jurisprudents, particularly Imam Khomeini, and also to study them critically from the viewpoint of Seyed Mostafa Khomeini.