when i was nechama leibowitz's (z"l) class at the hebrew university, she said that she didn't understand why rashi so frequently says: ein nah elah leshon bakasha. artscroll cites a radak who says that sometimes na means now (it also means raw, but i guess that's irrelevant). so perhaps rashi tells us when it means bakasha and not now.
dmartin, raanana eretz yisroel
Rashi says this seven times in Chumash (Bereishis 22:2, 38:25, 40:14, Shemos 11:2, Bamidbar 11:31, 12:6, Devarim 3:25). Though I have not looked at this moment at all seven, a quick glance at the commentaries will show you that Rashi is not telling you this to impart the rule that Na is a Lashon Bakasha, but rather he is proving that a specific explanation in the Pasuk has to be true due to this rule. [For examples, see Nachalas Yakov and Be'air ba'Sadeh on Bereishis 38:25, Gur Aryeh on Bamidbar 10:31, Sifsei Chachamim on Bamibar 12:6.]
All the best,
Yaakov Montrose