This paper deals with the problem of selecting service instances to execute workflow processes by not only taking into consideration Quality of Service (QoS) constraints, posed by the end-users, but also the business relationships between different service providers. These business / strategic relationships, the providers have, may affect the parameters of the service instances regarding the offered QoS level, and therefore the relationships need to be modeled and expressed with specific metrics. In this paper we present an innovative algorithm that maps workflow processes to service instances by taking into account the aforementioned metrics during the selection process. We also demonstrate the operation of the implemented algorithm and evaluate its effectiveness using a scenario, based on a 3D image rendering application.