Business-to-business workflow interoperation across Virtual Organisations (VOs) brings about possibilities for novel business scenarios. In such business scenarios, parts of workflows corresponding to different partners can be aggregated in a producer-consumer manner, making 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 stake holders at various levels in this hierarchy. This hierarchical SLA choreography and aggregation poses new challenges regarding its description, management, maintenance, validation, trust and security. In this paper we focus on the design and assessment of an agentenabled, rule-based validation framework for the hierarchical SLA aggregation, corresponding to cross-VO workflow cooperation.