Interfacial charge transfer in oxide heterostructures gives rise to a rich variety of electronic and magnetic phenomena. Designing heterostructures where one of the thin-film components exhibits a metal-insulator transition opens a promising avenue for controlling such phenomena both statically and dynamically. In this work, we utilize a combination of depth-resolved soft x-ray standing-wave and hard x-ray photoelectron spectroscopies in conjunction with polarization-dependent x-ray absorption spectroscopy to investigate the effects of the metal-insulator transition in LaNiO₃ on the electronic and magnetic states at the LaNiO₃/CaMnO₃ interface. We report a direct observation of the reduced effective valence state of the interfacial Mn cations in the metallic superlattice with an above-critical LaNiO₃ thickness (6 unit cells, u.c.) facilitated by the charge transfer of itinerant Ni 3d e_g electrons into the interfacial CaMnO₃ layer. Conversely, in an insulating superlattice with a below-critical LaNiO₃ thickness of 2 u.c., a homogeneous effective valence state of Mn is observed throughout the CaMnO₃ layers due to the blockage of charge transfer across the interface. The ability to switch and tune interfacial charge transfer enables precise control of the emergent ferromagnetic state at the LaNiO₃/CaMnO₃ interface and, thus, has far-reaching consequences on the future strategies for the design of next-generation spintronic devices.