The pasuk at 4:22 says: “וְאָמַרְתָּ אֶל פַּרְעֹה כֹּה אָמַר ה’ בְּנִי בְכֹרִי יִשְׂרָאֵל”- “You shall say to Pharoah, so says G-d: My firstborn son is Israel”
Rashi writes: “ומדרשו כאן חתם הקב”ה על מכירת הבכורה שלקח יעקב מעשו” - “Here G-d had agreed to the fact that Yaakov had taken the firstborn right from Esav”.
What is the connection here?
The Lev Aryeh explains with the following: The next Pasuk, (verse 23) writes that eventually a plague would come in which the firstborn of each Egyptian household would die.
If we look at Makas Bechoros (plague of the firstborn) in Parshas Bo (12:30) it says: ”כִּי אֵין בַּיִת אֲשֶׁר אֵין שָׁם מֵת” – “In every house there was a death”
Rashi asks, how could every household have a firstborn son? He goes on to explain that the Egyptian wives would commit adultery and have sons with other men, each of these sons would be the oldest to its father, hence when the Torah said that the firstborn would die, it referred to the firstborn of the father not the mother.
“דבר אחר מצריות מזנות תחת בעליהן ויולדות מרווקים פנויים, והיו להם בכורות הרבה, פעמים הם חמשה לאשה אחת, כל אחד בכור לאביו”
Now look at the Pasuk about the famous birth of Yaakov and Esav, in Parshas Toldos (25:26): “וְיָדוֹ אֹחֶזֶת בַּעֲקֵב עֵשָׂו וַיִּקְרָא שְׁמוֹ יַעֲקֹב”- “His hand was holding unto the heel of Esav, and they called him Yaakov”
Rashi writes:
“נמצא עשו הנוצר באחרונה יצא ראשון, ויעקב שנוצר ראשונה יצא אחרון, ויעקב בא לעכבו שיהא ראשון ללידה כראשון ליצירה, ויפטור את רחמה, ויטול את הבכורה מן הדין” – “Yaakov was created first, but was only born second, so he claimed that he deserves the firstborn rights, because he was created first, therefore he clutched onto the heel of Esav as if to hold him back.”
Rashi explains that Yaakov was created first; i.e. he was the firstborn of his father, but Esav; who was born first, was firstborn to his mother.
So from the plague of the firstborn where we see that “firstborn” refers to the firstborn of the father, and we can bring a proof that Yaakov deserved the right of the firstborn, as he was also firstborn of his father. That is the link with our original Rashi, showing because of Makas Bechoros, Yaakov was the true firstborn, in that Hashem passed judgment based on the formula Rashi presented at Yakov’s birth of being the father’s first son.
Amazing!