I have never heard it doubted that where a mortgagee sells, and has a balance in his hands, he is a trustee of that balance for the persons beneficially interested. He takes his mortgage as a security for his debt, but, so soon as he has paid himself what is due, he has no right to be in possession of the estate, or of the balance of the purchase-money. He then holds them, to say the least, for the benefit of somebody else, of a second mortgagee, if there be one, or, if not, of the mortgagor. What, then, is he to do? Surely he has a duty cast upon him. His duty is to say, 'I have paid my debt: this property which is pledged to me, and in respect of which I now hold this surplus in my hands, is not my property. I desire to get rid of this surplus, and hand it back to the person to whom it belongs.' ... The duty of this mortgagee was at least to set this money apart in such a way as to be fruitful for the benefit of the persons beneficially entitled to it. To that extent and in that manner he was, according to my understanding of the law, in a fiduciary relation to the persons entitled to the money. ...