Nazir 41a - Aseh Pushing off Lo Ta'aseh

Reish Lakish says that aseh pushing off a lo ta'aseh only applies when it is impossible to fulfill both, but if it is possible to fulfill both then aseh does not push off the lo ta'aseh. There is a big machlokes (starts with a Rabbeinu Yerucham and Beis Yosef in Hilchos tzitzis - i discussed it at length in my sefer) regarding situations where there isn't an inherent contradiction but the circumstances are such where the individual cannot fulfill the aseh unless he would violate the lo ta'aseh. There is a strong raya from the gemara in shabbos 133a that whenever the situation makes it impossible to fulfill both, one is allowed to push off the lo ta'aseh to fulfill the aseh, even though there isn't an inherent contradiction between the two. However the gemara here strongly implies the opposite. The gemara says that from the fact that we need "zekano" to teach that one violates giluach hazakan for the mitzvah of metzorah, indicates that metzora is b'ta'ar. The gemara suggests that maybe the metzorah can fulfill his mitzvah without a ta'ar but he has the right to choose a ta'ar. The gemara pushes that off based on Reish Lakish, that if it would be possible to reconcile the l'av of giluach hazakan with the shaving of metzorah, we would never darshen "zekano" to permit a metzorah to shave with a razor, therefore from the fact that we do darshen "zekano" to permit shaving with a razor the giluach of metzorah must be with a razor. The difficulty with this gemara is that perhaps a metzorah can fulfill his mitzvah even through plucking with a tweezers, but if he doesn't have tweezers and only has a ta'ar we need the pasuk to permit him to violate the issur of giluach hazakan. From the fact that the gemara doesn't entertain this possibility implies that Reish Lakish's rule that if it would be possible to fulfill both, we would not darshen "zekano" to be matir giluach hazakan with a ta'ar, would include even a case where an individual is in a situation where he can't fulfill both. Meaning, whenever there isn't an inherent contradiction where the lo ta'aseh prevents the fulfillment of the aseh, just a circumstantial contradiction (such as one who has only a razor and not tweezers), it is still considered "efshar l'kayem shneyhem" so that the aseh doesn't push off the lo ta'aseh (and we would not darshen "zekano" to be matir giluach).

To reconcile the gemara in shabbos with this very solid proof, see the tiferes yosef in the back of the gemara on nazir for an extremely lomdush approach to that gemara about milah b'tzara'as.