The extension of the legal effect of unauthorized (fuduli) sale to other contracts and unilateral legal acts remains a controversial issue in Islamic jurisprudence and Iranian civil law. Although the theory of “non-effectiveness” (suspension pending ratification) is widely accepted in the case of fuduli sale, its application to other legal acts still lacks a coherent and systematic framework, and existing responses are often fragmented and case-specific. The central question of study is whether the basis for non-effectiveness in a fuduli sale lies in the intrinsic nature of the contract of sale itself or in the absence of authority or permission at the time of formation. Accordingly, the research examines the extent to which this rule may be extended to other contracts and unilateral acts. Using a descriptive–analytical method and a comparative approach across Imami jurisprudence, the four Sunni schools, and Iranian positive law, this study aims to formulate a general criterion governing the extension or limitation of the fuduli doctrine. The findings indicate that the underlying rationale for non-effectiveness is the absence of authorization at the time of conclusion, rather than the specific nature of the sale contract. Therefore, the default rule supports extending this doctrine to contracts that are capable of representation and are not strictly personal in nature. However, in highly personal unilateral acts—most notably divorce—the extension of the fuduli doctrine is rejected due to their essential dependence on the authentic will of the right-holder. Consequently, the fuduli doctrine should be understood as a general yet qualified principle.