<p>In an increasingly competitive global market, supply chain efficiency has become a strategic priority for firms aiming to maximise profitability. Effective coordination among supply chain members, particularly through contractual agreements, plays a crucial role in optimising overall supply chain performance. This study develops an optimisation model for decision-making in a three-level dual-channel supply chain, explicitly incorporating retailer financial heterogeneity and collection constraints. The model adopts a Stackelberg game structure in which the manufacturer acts as the leader, while financially strong and weak domestic retailers, a foreign retailer, and a collector operating exclusively in the domestic market serve as followers. Using a Stackelberg game framework and numerical optimisation technique, this study analyses optimal pricing, delay in payment, sales efforts, and return mechanisms across five different collaboration scenarios. Numerical examples and sensitivity analyses are conducted to assess the impact of financial constraints, trade credit policies, and collection contracts on supply chain profitability. The results indicate that incorporating financially heterogeneous retailers and a collector significantly enhances both manufacturer and total supply chain profits. While financially strong retailers leverage payment delays to boost demand, weak retailers rely on service differentiation. In addition, the product return mechanism contributes to long-term cost savings and environmental sustainability. The novelty of this study lies in the integrated modelling of financial heterogeneity, closed-loop collection, and multi-channel coordination—an approach that has been rarely addressed in prior literature. The results provide actionable insights for manufacturers to design coordination contracts that are consistent with retailers’ financial capacities and dynamic market conditions.</p>