Bava Batra 137: Gifts with Conditions

The rabbis discuss gifts that are given with conditions attached.  For example, a person might say that his property is given to one person and when that person dies, it is given to another person.  That third person is permitted to acquire the property from another person if it was gifted or sold to another person erroneously.  The rabbis discuss how far these conditions might be taken.  Is it always possible to police gifts after they were re-gifted?  What about variations on this theme?  

This conversation turns to the timing of a receiving a gift.  If one gives a gift on his deathbed, is the gift acquired immediately or must acquisition wait until after the one on his deathbed has actually died?  And would acquisition take place at the moment of death (Abaye) or after death (Rava)?  This argument is compared with a husband who gives his wife a get with the condition that he dies first.  In such a case, the get is not valid.  

What if a person gives another person an etrog as a gift, and specifies that the etrog belongs to someone else after the receiver dies?  If the etrog cannot be used until after a person has died, then the etrog is not being used for its purpose and thus its owner has transgressed.  What if orphans bought an etrog with the money they inherited and one of them took it with the lulav to fulfill the mitzvah of lulav, but the others must permit him to eat it to demonstrate acquisition?  As part of this discussion, the rabbis agree that it is valid to give a gift and have it used (for example the etrog used for its mitzvah) and then return it. 

The rabbis share another example.  When Leah took fruit from her date tree located on Rav Bivi bar Abaye's property, he would get upset.  She transferred ownership to him for the rest of his life, and he bequeathed the tree to his son when he died.  The rabbis are appalled.  Of course this is not permitted!  One must specify the specific conditions around transferring ownership.  How specific would a person have to be to ensure that a gift is returned at a certain point in time?

At the very end of our daf, the rabbis wonder about an unwanted gift.  Must he acquire it?