TT #49 · Parashas Ki Seitzei · First aliya · Passuk 21:16
Then it shall be, on the day he causes his sons to inherit that which he has, that he may not make the son of the beloved the firstborn before the son of the hated, the firstborn.
Torah Temimah points to Sifrei Devarim.
English: “He may not” — one might have thought he is merely not permitted, but if he did prefer the younger the preference would stand; therefore Scripture says “he may not” (the act is void even after the fact).
Derash establishing that the forbidden preference is void ab initio.
An attempt to transfer the firstborn’s double portion to a younger son is void; the true firstborn keeps the share even if the father tried to prefer another.