Must an agent swear that his principal has not dismissed him from agency?

Primary text

If the party against whom the claim is made states, 'The principal has dismissed you (the agent),' and then demands that the agent swear, 'Swear that the principal has not dismissed you,' the agent is not made to swear. This is because the claim is directed against the principal, and the taking of an oath (Yamin) cannot be done through delegation (Niabah).