Kaisipang Hudyo sa Genesis 27:38: Derashot HaRan

Now if you will ask: Why should the blessing avail for Jacob, and why did Isaac say (Ibid 37): "But I have made him your lord, and I have given him all of his brothers as servants … and what shall I do now for you, my son!" How could he have made Jacob a lord if he had intended to give all to Esau! Did we not learn in respect to vows (Nedarim 11:5): "If his wife made a vow and he thought his daughter made it, or if his daughter made a vow and he thought his wife made it … he must annul it again," for what is said to one person on the assumption that he is a different person does not avail him at all. This being so, why should Isaac have said "and what shall I do now for you my son?" Furthermore, why did Esau not challenge the blessing itself instead of merely protesting (Ibid 38): "Do you have just one blessing, my father?" He should have said: "That blessing was given in error and will not avail him; give it to me!"