In this poster, we start the discussion of the potentials and challenges of digital payment systems to advance digital services in the Internet of Things. We specifically focus on devices with constrained hardware resources. To enable multi-stakeholder machine-to-machine scenarios, we propose an e-cash approach that is privacy-friendly and allows for autonomous payment. We implement our approach using GNU~Taler, a free-software e-cash implementation, and RIOT, a free and open-source operating system for the IoT. Our preliminary findings suggest that the deployment of e-cash systems is feasible in constrained IoT scenarios. They underscore the importance of concise, standard-compliant data encoding over computationally intensive compression techniques.