If you want to give a unique domain to a different partner or want to stop using a certain domain, you can revoke a partner's ability to use a unique domain.
From the left navigation bar, select Content → Unique Domains.
Under Actions, select [Drop-down menu] → View Assigned Unique Domains.
Find the unique domain you want to revoke partner use on.
Under Actions, select Revoke.
Read the warning message, then select I understand, continue.