Where the agent has himself an interest in the property which forms the subject-matter of the agency, the agency cannot, in the absence of an express contract, be terminated to the prejudice of such interest.
     Illustrations
     (a) A gives authority to B to sell A's land, and to pay himself, out of the proceeds, the debts due to him from A. A cannot revoke this authority, nor can it be terminated by his insanity or death.
     (b) A consigns 1,000 bales of cotton to B, who has made advances to him on such cotton, and desires B to sell the cotton, and to repay himself out of the price, the amount of his own advances. A cannot revoke this authority, nor is it terminated by his insanity or death.