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.
English: “He may not” — Rabbi Elazar said: what does “he may not” mean? He is not permitted. And when is he able? By means of a gift.
On the daf / in context: A father may not simply declare that the younger son of the beloved wife is the firstborn. Such a declaration is void. He may, however, give property as an outright gift during his lifetime, which is effective.
Type of Learning
Derash distinguishing forbidden preference from a valid gift.
What Is Learned
A father may not re-label the firstborn by declaration, but he may transfer property by a valid gift.