For the Service Economy to prosper, IT-based Service Markets are required to perform certain business actions autonomically and autonomously, e.g., helping companies to establish networks of business relationships. Service Markets, to be practically realized require an enabling infrastructure that supports business-to-business (B2B) relationships among business stakeholders, resulting in value chains. B2B workflow interoperation across Virtual Organization (VOs) brings about novel business scenarios. In these scenarios, parts of workflows corresponding to different partners can be aggregated in a producer-consumer manner, leading to hierarchical structures of added value. Service Level Agreements (SLAs), which are contracts between service providers and service consumers, guarantee the expected quality of service (QoS) to different stakeholders at various levels along this hierarchy. Automation of service composition in these coalition workflows directly implies the aggregation of their corresponding SLAs. This hierarchical choreography and aggregation poses new challenges regarding SLA description, management, maintenance, validation, trust, and security. In this paper, we focus on design and architecture of an agent-oriented, rule-based validation framework for hierarchical SLA aggregation, enabling cross-VO workflow cooperation. The framework is based on the Rule Responder architecture, the RBSLA project, a formal model of SLA Views, and a distributed trust model.