Grid computing empowers users to employ a huge amount of resources to perform their task. Many services allows to access these resources with different methods and supporting different QoS. In order to use the services an agreement between the user and the provider has to be de- fined, where users specify the QoS required from the service to perform their activities correctly and, at the same time, providers declare the QoS offered with the service. Current solutions to reach an agreement between users and providers do not consider many aspects related to the use or misuse of services that can seriously influence or even nullify the agreement itself. This paper presents a simple formalism to define more rigourously an agreement, and a discussion about several