Bechorot 52 - Firstborn Gets Double
Firstborn son gets a double portion in the inheritance from his father's estate, but not from his mother's. The Torah said, " to him applies the law of the firstborn ." Even though it is talking here about him, the son, it also hints that this law applies only to "him", the father, and not to "her", the mother.
If some time passed between the death of the father and the division of the estate, and the property has meanwhile improved (fruit grew on trees), the firstborn does not get a double share in those improvements, but only a regular one, like the rest of the sons. Of course, when he gets the double share of the land, it may happen that together with it he will double share of the improvements, but he will have compensate his brother for them. The Torah said that he gets double from " all that is found in his father possession " to tell us that he does not get from what comes to the estate later, after death. He also does not get a double portion from the money owed to his father but uncollected at the time of death.
Art: Bartolome Esteban Murillo - The Prodigal Son Receiving His Portion Of Inheritance