Non est in facultate mandatarii addere vel demere ordini sibi dato

It is not the function of a mandatary to add or subtract from the authority which has been given to him. "He (the mandatary) was not authorized to exchange the lands for other property, or to accept the notes of the vendee as cash, or to accept personal security, or any form of security except that specified in the condition." Morrill v Cone (US) 22 How 75, 16 L Ed 253, 255.