27:4 Why should the name of our father be done away from among his family, because he had no son? Give unto us a possession among the brethren of our father.'
The implication being that had their father possessed a son, that son would receive the inheritance due to the father and not the daughters. The gemara disposes of this proof by stating that we cannot learn from them because "nitnah Torah v'nitchadsha halachah" - we cannot learn halachic proofs from situations occurring before the giving of the Torah. The commentary on the page states that this is referring to the later parshas nachalos in that very chapter (and disposed of as a proof earlier on the daf). However, that parshah is clearly stated as a direct response to the claims of B'nos Tzelofchad and confirms their understanding of how the inheritance process should operate.
On its face, the concept of "nitnah Torah v'nitchadsha halachah" seems to be a strange response. According to Rashi at the beginning of Parshas Behar (quoting Toras Kohanim), Chazal understand that all particulars of the Torah were transmitted at Sinai. The story in question clearly took place at the end of the 40 years in the wilderness, and thus long after Matan Torah.
One option that remains is that while the corpus of law was given orally at Sinai, the completion of the narrative happened only upon the death of Moshe, which has implications regarding how the written Torah was given (whole vs. piecemeal). It would also mean that NO story in Chumash can be considered halachically definitive, since they all take place before the canonization of the written Torah.
Does anyone provide a clarification of the concept of "nitnah Torah v'nitchadsha halachah" that grapples with some or all of the above points?