"he shall not grant primogeniture": We are hereby taught that he is not permitted to grant it. I might think that he is not permitted to do so, but if he did so, it stands; it is, therefore, written "he shall not be able to grant primogeniture," (the implication being that) if he does, it shall not stand.
"… over the face of the son of the hated one": We are hereby taught that if his head or most of it emerged live (from the womb), he negates the next birth as a first-born.
(דברים כא טז) וְהָיָה בְּיוֹם הַנְחִילוֹ אֶת בָּנָיו, בַּיּוֹם מַפִּילִים נַחֲלוֹת וְאֵין מַפִּילִים נַחֲלוֹת בַּלַּיְלָה.
(Devarim 21:16) "Then it shall be on the day that he causes his sons to inherit": Inheritances are adjudicated in the daytime and not at night.
אֵת אֲשֶׁר יִהְיֶה לוֹ, מְלַמֵּד שֶׁהַבֵּן נוֹטֵל בָּרָאוּי כְּבַמֻּחְזָק.
"what there shall be to him": We are hereby taught that the son inherits what is anticipated as well as what is on hand.
לֹא יוּכַל לְבַכֵּר, מְלַמֵּד שֶׁאֵינוֹ רַשַּׁי לְבַכֵּר. יָכוֹל לֹא יְבַכֵּר, וְאִם בִּכֵּר – יְהֵא מְבֻכָּר? תַּלְמוּד לוֹמַר: לֹא יוּכַל לְבַכֵּר, הָא אִם בִּכֵּר – אֵינוֹ מְבֻכָּר.
"he shall not grant primogeniture": We are hereby taught that he is not permitted to grant it. I might think that he is not permitted to do so, but if he did so, it stands; it is, therefore, written "he shall not be able to grant primogeniture," (the implication being that) if he does, it shall not stand.
אֶת בֶּן הָאֲהוּבָה, כֵּיוָן שֶׁיָּצָא רֹאשׁוֹ וְרֻבּוֹ בַּחַיִּים – פּוֹטֵר אֶת הַבָּא אַחֲרָיו מִן הַבְּכוֹרָה.
"… over the face of the son of the hated one": We are hereby taught that if his head or most of it emerged live (from the womb), he negates the next birth as a first-born.
עַל פְּנֵי בֶן הַשְּׂנוּאָה הַבְּכֹר – אַף עַל פִּי שֶׁבֵּן הַבְּכוֹר לַשְּׂנוּאָה.
"...in disregard of the son of the unloved one who is older": Although the older is of the unloved one.