Bava Batra 114: Limiting a Son's Maternal Inheritance

When can the respective parties renege on a formal act of acquisition (kinyan)? Of course, it's a machloket -- whether they can walk it back for as long as they're sitting together or whether they must still be talking about the transaction (regardless of whether they are still sitting together). Also, a woman bequeaths to her son, but doesn't inherit from her -- but this case seems redundant in the mishnah! The Gemara explains that just as the husband inherits some categories of property from his wife, but not no matter what -- so too, her son. The restating of the case specifies that the son too doesn't inherit from his mother's posthumously acquired property.