When Yaakov Avinu asked Eisav to sell the rights of the firstborn, he uses the extra word “today.” There are many explanations to this, and the Midrash HaGadol uses this word to learn an interesting Halacha. The Midrash states that Yaakov Avinu was saying that the purchase of the rights of the firstborn only works “today,” but after Mattan Torah, this type of sale will not work.
Rabbi Aharon Dovid Singer
Rosh Kollel, NMB Kollel