In the Grid computing model a remote service is provided by a resource owner to a client. The resource owner executes a client job and charges the client for a corresponding fee. In this paper we discuss the main weakness of many existing models for performing such a kind of transaction, i.e., the strong assumption that both the resource owner and the clients are honest. Then, we propose a new security model in which either the resource owners or the clients (or both) may not be honest. Our model introduces a trusted third party, referred to as “Grid Manager”. We describe in details the role of the Grid Manager and argue the advantages of our proposal with respect to the current state-of-the art.