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Background. Open Source Software (OSS) remains underfunded, threatening the sustainability of critical digital infrastructure. In response, several public, private, and philanthropic funding initiatives are emerging. However, demand greatly exceeds supply, underscoring the need to increase funding and improve the efficiency and impact of existing funding sources. Aims. This study investigates how OSS funders conceptualize, measure, and report funding impact, and the implications for sustaining and funding OSS projects. We focus on how funders select projects, define objectives, measure outcomes, and justify continued funding. Method. We conducted a qualitative study based on semi-structured interviews with 25 participants representing funders, funding recipients, and research organizations in the OSS ecosystem. The findings were synthesized using thematic analysis and validated and enriched through a follow-up workshop with 16 participants from overlapping organizations. Results. We find that impact is typically measured through short-term, milestone-based deliverables and qualitative narratives, and is dependent on personal funder-recipient relationships. Long-term sustainability, risk mitigation, and human-centered impacts are difficult to capture and rarely measured systematically. Measurement practices are shaped by (often implicit) funder strategies, funding structures, and reporting requirements, leading to ad-hoc and project-specific approaches. Conclusions. Measuring and reporting funding impact remains a shared and non-trivial challenge for OSS funders. Addressing this requires explicit impact logics and shared meta‑frameworks (rather than universal metrics) to enable a more coordinated and complementary OSS funding ecosystem. This, in turn, can improve the effectiveness, accountability, and availability of funding and ultimately support the long-term sustainability of OSS as critical infrastructure.