The Supreme Court’s Unifying Decision No. 847, dated May 14, 2024, addresses the obligation of attorneys to observe the best interests of their principals in agency-based transactions. However, it pays little attention to the rights of bona fide third parties. Although the decision emphasizes the necessity of protecting the principal’s interests, its ambiguity regarding transactions involving third parties acting in good faith has raised doubts about transactional security and judicial consistency. The present study aims to analyze the legal implications of this decision for the validity of transactions against bona fide third parties and to clarify the criteria for establishing good faith. The research adopts a descriptive–analytical method, drawing upon Islamic jurisprudence, statutory law, and judicial precedents, with a critical approach toward existing case law. While Unifying Decision No. 847 suggests that even transactions with bona fide third parties may be voidable for lack of observance of the principal’s interests, the doctrine of constructive destruction (telef hukmi) has emerged in judicial practice as a solution to safeguard the rights of bona fide third parties. This approach has also been reinforced in the 2024 Law on Mandatory Registration of Real Estate Transactions. The findings of this study contribute to strengthening transactional security and to developing clearer rules for determining the good faith of third parties.