Advances in computing and networking technology, and an explosion in information sources has resulted in a growing number of distributed systems getting constructed out of resources contributed by multiple sources. Use of such resources is typically governed by sharing agreements between owning principals, which limit both who can access a resource and in what quantity. Despite their increasing importance, existing resource management infrastructures offer only limited support for the expression and enforcement of sharing agreements, typically restricting themselves to identifying compatible resources. In this paper, we present a novel approach building on the concepts of tickets and currencies to express resource sharing agreements in an abstract, dynamic, and uniform fashion. We also formu-late the allocation problem of enforcing these agreements as a linear-programming model, automatically factoring the transitive availability of resources via chained agreements. A case study modeling resource sharing among ISP-level web proxies shows the benefits of enforcing transitive agreements: worst-case waiting times of clients accessing these proxies improves by up to two orders of magnitude.