The Cash-Out Boundary: A Systems and Economic Framework for Affordable Mobile-Money Ecosystems in Sub-Saharan Africa

Mobile-money systems are commonly assessed through adoption, transaction volume, and nominal fees. These measures can conceal a deeper design problem: digital value may remain useful only until a participant must convert it into cash. This paper introduces the cash-out boundary, defined as the point at which a digital balance can no longer complete an intended economic action at an acceptable cost, reliability, or level of trust. The paper argues that reduced consumer withdrawal creates genuine savings only when merchants, suppliers, workers, and service providers can also reuse digital balances rather than inherit equivalent conversion costs. It proposes four analytical measures: the Cash-Out Dependency Index, Digital Liquidity Retention Ratio, Digital Liquidity Depth, and Effective Cost Ratio. A directed, weighted, temporal graph model and an open-source simulation architecture are proposed to examine how merchant acceptance, interoperability, pricing, network reliability, and agent liquidity affect system-wide cost. The framework distinguishes genuine cost reduction from the transfer of cash-conversion costs to downstream participants. The charts in this preprint are illustrative analytical scenarios and are not presented as empirical findings. The paper has not yet undergone peer review.