172. Authority coupled with interest.

Where the agency is created by deed, or for valuable consideration, and the authority is given to effectuate a security or to secure the interest of the agent, the authority cannot be revoked1. Thus, if an agreement is entered into on a sufficient consideration whereby an authority is given for the purpose of securing some benefit to the donee of the authority, the authority is irrevocable on the ground that it is coupled with an interest2. So an authority to sell in consideration of forbearance to sue for previous advances3, an authority to apply for shares to be allotted