Cloud Computing is a new paradigm which provides ubiquitous access to shared computing resources for organizations and end users. In this paper, we propose an agent-based mechanism to automatically manage Cloud federations and requirements change to accept the maximum of requests with minimum cost and energy consumption. First, we present the mechanism's strategies: gain strategy, acceptance strategy and offer strategy. During the implementation of the mechanism (or after a certain time) the user's needs may change (some are modified, others are added or deleted). So, we present how to manage the requirements change of the previous strategies. Finally, simulation results indicate that these policies enhance the provider's profit.