Bava Batra 126: Firstborns Changing Their Minds; Who was Born First?

In amud (a), the rabbis continue to discuss the ins and outs of the firstborn's birthright.  We are told that the firstborn does not receive a double portion based on improvements made by brothers to the father's land after the father has died.  If the firstborn decides to divide the land evening and each improves his own property, then he will receive a double portion of the improved lands.  

Is this always true?  If the grapes or olives were improved, are they counted?  If the grapes or olives were simply harvested, can that "improvement" be part of the double portion? Does the forfeiting have to be stated in advance? What if the grapes were changed to wine?  Which is more valuable, grapes or wine?  And should that be a factor in whether or not the firstborn has access to his double portion when they are included?

Rav Asi posits that a firstborn son who takes a regular portion has forfeited his extra share.  Rav Papa, in the name of Rava, answers that one cannot pardon that which he does not already own, and thus the firstborn cannot forfeit his extra share until the field has been divided.  Rav Papi teaches that Rava also sad that the firstborn pardoned his extra share when it comes to all of his property.  Rava was said to have called a sale void when a firstborn sold his entire property and then his brothers, orphans, were hit by the new owner when they attempted to eat the dates from what was their field.  Based on the fact that the property was not yet divided and thus not yet belonging to the firstborn, the sale was null.  Rav Papi and Rav Papa disagree as to whether the entire sale was voided or whether the firstborn was permitted to sell his own share.  

A new Mishna teaches that if a father states that his firstborn (with his name included) will not receive a double portion or will not inherit with his brothers, this goes against Torah law and will not be in effect. If a person who believes he is about to die states how much each son should receive and the amounts are unequal or the firstborn's shares are equal, this will take effect.  If he writes this stipulation anywhere in a document, it takes effect.  However, if this was their inheritance, it does not take effect.  The Gemara considers other situations, like kiddushin, where words might cause one to change a vow and effectively dispute Torah law.

What happens when we do not know which son is actually the firstborn?  If a man says that a particular son is his firstborn, is that proof enough?  What if that son was the first born to a particular mother?  And what if a witness saw that another son was in fact the firstborn?  One argument is that the witness saw that this son was first born to one particular mother.   His father would call him "foolish firstborn" in such a case.  And we learn that a father might prove that one is his firstborn because it was witnessed that the father would call that son to use his spit to heal another person's eye aches.  This healing trait was thought to travel from father to firstborn son.

The rabbis wonder about a tumtum, one who's sex is indeterminate because his genitals are obscured. Does a tumtum revealed to be male inherit the double portion?  We are told that he must be demonstrably male from the time of his birth to be eligible to hold the status of firstborn son and the benefits that role assumes.