Baba Basra 116b - Understanding the Concept of Ra'uy

The Rashbam explains that if yacov would die in the lifetime of his father yitzchok, Reuven (son of yacov) would only get the double portion in the estate of yacov his father, but not in the estate of yitzchok his grandfather because it is only considered "ra'uy". Tosafos disagrees with the Rashbam because Reuven has absolutely no claim on a double portion in the estate of Yitzchok. Since yacov himself is not a bechor and isn't entitled to double in the estate of yitzchok, when Reuven comes to inherit through yacov he also doesn't deserve a portion even if a b'chor is entitled to get what is ra'uy. Rather, we consider ra'uy the regular portion that yacov would be inheriting b'kever from yitzchok - Reuven wouldn't get a double portion in the chelek pashut that yakov passes on from yitzchok since yacov was never muchzak in that portion.

Regarding the determination of what is considered ra'uy, the ketzos (278:15) discusses a case where someone takes an oath to give a gift to yacov, but yacov died before it was given. Does Reuven take a double portion in that? He proves it from matnos kehuna which the gemara considers the father (who is a kohen) to be muchzak in (when he is makirei kehuna) even though he doesn't have it yet. The idea is that even things which the father doesn't presently own at the time of his death, if they will be definitely coming to him because such as the case of shavua or matnos kehuna, where it is assur for the giver to back out, we consider it to be muchzak.

The Steipler (38) uses this idea to explain the shitah of the Rambam. Based on the kesef mishne (nachalos 5:6) quoting the Teshuvos HaRosh to explain the Rambam, the steipler says that the Rambam would seem to hold that if Yacov would have 10 children, but 5 of them died in his lifetime (and they don't have children), we view it as if the remaining 5 brother inherit half the estate from their father and the other half from their brothers who died. Based on this, when Reuven comes to collect the portion of bechor from the estate of yacov, we divide the estate 11 ways, and give Reuven only 1 portion for as a bechor. Then we should take the remaining 10 portions and divide them equally among the remaining 5 brothers, so that each will take 2 portions. Yet, we don't say this, rather we allow Reuven to collect his bechor portion from the entire estate - why? Shouldn't the portion that he inherits through his deceased brothers be considered ra'uy? From here we see that anything which is set to come to the father is not considered ra'uy, so the portions of the 5 sons who died is considered muchzak by the father so that Reuven can collect his bechor portion from that.