Ḥifẓ al-bīʾah is increasingly proposed as an independent maqṣad within Maqāṣid al-Sharīʿah, yet claims of convergence with indigenous environmental custom are rarely tested against a differentiated standard. This article addresses that gap through the Ammatoa Kajang community of Bulukumba, South Sulawesi, whose forest conservation is governed by Pasang Ri Kajang. Rather than assuming harmonization, this study develops a three-level framework surface value similarity, functional convergence, and normative resonance, to specify how far the two systems align. Data were collected through a qualitative case study combining participatory observation and in-depth interviews with fifteen informants: customary leaders, religious figures, and community practitioners. Findings show two patterns: Pasang's ethical-normative dimensions converge functionally with Islamic principles such as zuhd and amānah, while its cosmological dimensions remain in tension with doctrine on tawḥīd, negotiated unevenly by informants’ institutional proximity to formal Islam. The study contributes empirically by showing Pasang Ri Kajang is not an undifferentiated whole; theoretically by arguing ḥifẓ al-bīʾah warrants maqṣad status because theological internalization, not fiqhenforcement, sustains conservation; and sociologically by showing da’wah toward customary communities succeeds through cultural accommodation and negotiated reception rather than confrontation.