Scrum in Agencies: Where Does the Product Owner Live?

Implementing Scrum in a standalone organisation (such as a product company) derives directly from the framework. Yet some aspects are open to interpretation for Scrum teams in an agency providing software development services to clients. Both The Scrum Guide and most literature on best practices of implementing Scrum lack the notion that the so-called “organisation” may be, in fact, two separate legal and business entities: the agency and the client. How do you apply The Agile Manifesto’s “Customer collaboration over contract negotiation” when the underlying “organisation” exists precisely because of a contract?