וְהָיָ֗ה בְּיוֹם֙ הַנְחִיל֣וֹ אֶת־בָּנָ֔יו אֵ֥ת אֲשֶׁר־יִהְיֶ֖ה ל֑וֹ לֹ֣א יוּכַ֗ל לְבַכֵּר֙ אֶת־בֶּן־הָ֣אֲהוּבָ֔ה עַל־פְּנֵ֥י בֶן־הַשְּׂנוּאָ֖ה הַבְּכֹֽר׃
ay mangyayari na sa araw na ipamamana niya sa kanyang mga anak na lalaki ang kanyang mga ari-arian, hindi niya maaaring ituring na panganay ang anak ng minamahal sa halip na ang anak ng kinapopootan, na siyang tunay na panganay.
Jerusalem Talmud Bava Batra
HALAKHAH: “One who says, “my firstborn X shall not take a double portion,’ ” etc. Rebbi La made the right of the firstborn equal to that of the brothers88He approved a will which gave the firstborn only what the other brothers received.. Rebbi Ḥaggai said to them: Is that not a verse, “he shall not be able to declare as firstborn.”89He thought that Rebbi La acted in error.
90Midrash Tanna'im (Midrash Haggadol Deut. 21:16); Sifry Deut. 216.Rebbi Eliezer said, by the Temple Service! He is able, but he is not permitted91What is missing here (compared to the other sources) is the statement that any action to deprive the firstborn of his inheritance be invalid. In general we hold that if a certain action is biblically forbidden, if it is done anyway it is valid in law since, if it were invalid, it could not be punishable. But an action by the father to deprive his firstborn of his double portion is not punishable; therefore, it is invalid.. Except that he is able designating it as a gift87In a document one has to follow the formal rules. If he used the language of gift but also mentioned inheritance, the will is valid as explained by R. Hoshaia in the Halakhah..
90Midrash Tanna'im (Midrash Haggadol Deut. 21:16); Sifry Deut. 216.Rebbi Eliezer said, by the Temple Service! He is able, but he is not permitted91What is missing here (compared to the other sources) is the statement that any action to deprive the firstborn of his inheritance be invalid. In general we hold that if a certain action is biblically forbidden, if it is done anyway it is valid in law since, if it were invalid, it could not be punishable. But an action by the father to deprive his firstborn of his double portion is not punishable; therefore, it is invalid.. Except that he is able designating it as a gift87In a document one has to follow the formal rules. If he used the language of gift but also mentioned inheritance, the will is valid as explained by R. Hoshaia in the Halakhah..