Bava Batra 126: Double Portion Tension

More on the double portion inherited by the first-born. What happens if the first-born renounces the extra part of his double portion, so that he ends up with a comparable inheritance to other brothers? Rav Pappa and Rav Pappi (yes) both weigh in, based on inference. Plus, an actual case that came before Rava adjudication. Also, a new mishnah! Namely, if the father attempts to decree a different kind of inheritance division than that which is established in the Torah - it won't work. Maybe if he reapportioned his property via gifting, but not inheritance. Plus, the question: first-born to whom? How do the different parents make a difference in this regard?