Molecular doping is a promising strategy to fine-tune the electronic properties of halide perovskites and accelerate their implementation in next-generation optoelectronics. However, a deeper understanding of the role of host-dopant interactions in these systems is needed to fully exploit the potential of this avenue. Herein, we demonstrate a surface post-treatment strategy employing n-type molecular dopant n-DMBI-H to modulate free hole density in p-type CH₃NH₃Sn_0.75Pb_0.25I₃ films. We show that the adsorption of n-DMBI-H on surface Sn atoms, followed by the dissociation of an electron-donating hydride from the dopant, facilitates charge transfer to the perovskite and hole trapping at the dissociated hydride. We identify this mechanism as a key factor dictating doping compensation in perovskites, allowing carrier density control within nearly 1 order of magnitude via the dissociated molecular dopant located at film surfaces and grain boundaries. We then exploit n-DMBI-H in perovskite/transport layer junctions, achieving reduced carrier losses and improved contact selectivity and performance in p-i-n, Sn-rich perovskite solar cells. We expect this work to provide carrier density tuning guidelines for a broad range of tin-based perovskite applications.