The Service Level Agreement (SLA) based grid superscheduling approach promotes coordinated resource sharing. Superscheduling is facilitated between administratively and topologically distributed grid sites via grid schedulers such as Resource brokers and workflow engines. In this work, we present a market-based SLA coordination mechanism, based on a well known contract net protocol. The key advantages of our approach are that it allows: (i) resource owners to have finer degree of control over the resource allocation which is something that is not possible with traditional mechanisms; and (ii) superschedulers to bid for SLA contracts in the contract net, with focus on completing a job within a user specified deadline. In this work, we use simulation to show the effectiveness of our proposed approach.