Parashas Bo · Shemos 12:9 · TT note 12
Torah Temimah on Shemos 12:9, note 12 (on כִּי אִם צְלִי אֵשׁ) points to Pesachim 41b.
Sefaria — Exodus 12:9
Sefaria — Torah Temimah on Exodus 12:9
Sefaria — Pesachim 41b
E-Daf — Pesachim 41b
English: Why does it say “but”? To tell you: at the time when one is under the positive command “arise and eat roast,” one is under the prohibition “do not eat underdone”; and when one is not under the positive command, one is not under that prohibition.
Linkage of lav to positive mitzvah: the ban on na applies only when the mitzvah of eating roast is in force.
If one ate an olive’s bulk of na outside the time of the eating mitzvah, the specific lav of na does not apply.