I  have a custom record representing licenses/contracts a customer has purchased. Customers don't have any reason to see these records in the system, but their PARTNERS do. If I'm a Partner, I want to be able to see what my customers own, but I sure don't want other partners to see what my customers own. FYI - we use the Advanced Partner Center.

To the point, the custom record definition has options to Show Owner on Record and to Allow Change. If I enable these, I (as Administrator) can change the owner to any EMPLOYEE users, but not customer or partner users (I want to set it to the Partner). However, if I log in as a Partner and create such a record, the Owner field is set to the Partner and can save that way (this permits the Restrict Viewing and Editing permission to work exactly as I want). I (as Administrator) can't change it to another user, Save, then change it back to the Partner; it behaves similarly to the idea of inactivating an employee who is set