In a virtual organisation (VO), a group of cooperating agents may offer resources (or services) consisting of all, or part, of the sum of their individual contributions. Some of these resources may already be in use for certain time periods (existing commitments), and these existing commitments may vary in value to the VO. Using constraint reification and cumulative scheduling methods, we investigate ways in which agents may manage their resources and existing commitments when faced with a decision to take on new commitments. We describe a technique that allows an agent to intelligently construct satisfiable permutations consisting of existing and new commitments, and use preference and quality information to choose between these permutations. We show how constraint reification is used to model whether commitments are breakable/re-negotiable and how this influences the permutations available to the agents.