As we have learned, the Torah commands (see 21:15-17) that the firstborn son will receive a double portion of the estate left by his father at the time of his death. What if the estate grows after the father’s death, but before the children have had time to divide it up into the shares that they will receive? Will the firstborn get a double portion in those additional profits?
According to the Tanna Kamma, the firstborn has no unique right to anything beyond what was actually in the estate at the time of the father’s death. The Gemara explains that this is derived from the passage that says that the father will “give him a double portion” ( 21:17) – “giving” is understood to mean that this is a type of present, and a person cannot give a present until he actually has possession of it. Based on this understanding, the firstborn cannot expect a double portion from anything that his father did not actually have in his possession at the time of his death.
Rabbi Yehuda HaNasi disagrees, arguing that the firstborn has rights to profits that the estate earned even after the father’s death. The source for this ruling, according to the Gemara is from the same passage, but Rabbi emphasizes the expression pi shenayim – double -which is understood to mean “two times of a single share,” so that whatever a single share would be is simply doubled for the firstborn.
The Ri Migash points put that even according to Rabbi’s opinion there is no suggestion that the firstborn would have special rights to money that comes into the estate after the father’s death – no one accepts that possibility. Rather this opinion is limited to that property that the father already had in his possession during his lifetime that increases in value after his death.