We develop a theoretical model of supplier finance where an intermediary (e.g., a large buyer) pools trade credit and allocates liquidity across heterogeneous suppliers. Optimal supplier finance creates profit-driven liquidity cross-subsidization and explains selective supplier inclusion as an equilibrium outcome. Surprisingly, higher funding costs can increase welfare by expanding supplier participation. The model also uncovers a novel non-monotonic relationship between operational efficiency and supplier finance adoption. Depending on financing costs, intermediation and liquidity provision act as either complements or substitutes, offering new insights into why some buyer firms adopt supplier finance while others do not.