How one nekuda makes a difference!
The Gemara is discussing the issue of the Korban of Pesach Sheni. The mishnah on the bottom of 64b had discussed what Korban Pesach Rishon and Korban Pesach Sheni have in comon and where they differ.
The Gemara commences the discussion of this mishnah with the following cryptic line:
“Kesiv [the reference in the Torah Or to Shemos is in error – as this is a discussion of Pesach Sheni, the reference should be to Bamidbar]: ‘Lo yashiru mimenu ad boker’ – Im le’echol, zeh mitzvas aseh she’bo. ‘V’etzem lo tishberu bo’ – zeh mitzvas lo ta’aseh she’bo.”
The second line is quite clear – the Gemara is explaining how we know that there is a prohibition to break the bones of the Korban Pesach Sheni. But what about the first line? The citation from the pasuk is clearly a prohibition, not a positive command? Furthermore, what is the word “Im” doing here? (The Korban Ha’Edah just ignores it. The Pnei Moshe tries to deal with it.)
The mefarshim have grave difficulty with the phrase. The Mashbiach goes so far as to suggest that “le’echol” here means to consume by fire and the Gemara means to say that since there is a prohibition to leave the Korban until morning, the lav of nosar, by Korban Pesach Sheni, there must also be an aseh, to burn the nosar, as nosar is a lav hanitak l’aseh! (See also the Yerushalmi k’Peshuto who has his own difficult explanation.)
It seems to me that the Yerushalmi can be explained very simply, not by changing any words, but by adding one dot, i.e., to read “eim” (with a tzerei) instead of “im” (with a chirik). The word eim in this context is the same as it is in the context of “yesh eim l’mikra, yesh eim l’masores.” It means that there is a source and a basis for understanding (“the mother of all understandings”).
Read thus, the Gemara is quite clear: If there is a prohibition to leave Korban Pesach Sheni until morning, that is a source and basis for the converse of that prohibition – where there is a lav on leaving, there must be an aseh on eating. Therefore, the same mitzvah to eat Korban Pesach Rishon must apply to Korban Pesach Sheni. Thus, the presence of the prohibition is the source and basis for the application of the mitzvas aseh to eat Korban Pesach to Pesach Sheni.