Enterprise resource planning (ERP) systems are often cumbersome to customize to a client´s needs. Model-driven approaches promise to simplify these attempts. In this work we present an ERP prototype based on the Resource-Event-Agent (REA) business ontology that follows a"model-at-runtime"approach: one may customize the ERP system during runtime by changing the underlying REA models. We are using Retail-H as the reference framework for building a retail information system (RIS). Our main contribution is the prototypical implementation of a domain agnostic REA engine that can be loaded at runtime with domain specific business models-these models can further be manipulated at runtime. On that basis we have exemplarily modeled main concepts of Retail-H in REA. Validation of the implemented components is realized by applying real business activities and requirements received from our partner, a business software solution provider.