Does the agency contract (Wakalah) terminate upon the principal's revocation or the agent's resignation?

Primary text

The agency contract is revocable by mutual consent; the principal may dismiss his agent at any time, and the agent may resign. This is because agency is a form of permission to act, which either party may invalidate, akin to permission to eat one's food. Termination also occurs upon the death or permanent insanity of either party, with no known disagreement on these points.