The integration of product recovery into regular production processes enables new opportunities for cost savings. It also yields new challenges for inventory management because there are two sources to satisfy product requirements which need to be coordinated by additionally taking into account limited availability of used products. In case of a dynamic planning situation, for instance when dealing with seasonality or the product life cycle, new motives for keeping stock arise. The work aims to identify those motives and to describe their effects by using methods of optimal control theory.