Complex sentence: The new treaty recognized that Casimir was the sole suzerain of Moldavia, prohibiting Stephen to alienate Moldavian territories without Casimir's authorization and obliging him to recapture the Moldavian territories that had been lost, which obviously referred to Chilia.
Split into simpler sentences: The new treaty declared that Casimir was the sole suzerain of Moldavia, prohibiting Stephen to alienate Moldavian territories without Casimir's authorization. The treaty also obliged Stephen to recapture the Moldavian territories that had been lost, obviously in reference to Chilia.