A method is proposed for acquiring a service sold over a data network (20) against presentation of a transaction medium (2). The service is ordered by a user (1) by a user end system (10) from a service provider (40). The latter calls in a clearing center (30) which enters into a data exchange with the transaction medium (2) to call for therefrom a necessary contribution in the form of certain data assumed for acquisition of the service. The ordering process and the data exchange with the transaction medium (2) for calling for the contribution are linked via an identifier (KM) associated with the transaction medium (2), the identifier being determined by the user end system (10) and used to secure the ordering process and the call for the contribution.