(a) In Insights to the Daf, Pesachim (35:1) the Kollel writes:
>>RABEINU DAVID explains that the principle of "Aseh Docheh Lo Ta'aseh" applies only when the Torah itself forces the Aseh and the Lo Ta'aseh into opposition. If the confrontation between the Aseh and the Lo Ta'aseh comes only as a result of one's particular situation and circumstances (for example, he has no other Matzah available other than Matzah of Tevel), then "Aseh Docheh Lo Ta'aseh" does not apply. In such a situation, the Torah does not tell the person to eat Tevel in order to fulfill the Mitzvah.
(This is in contrast to other instances of "Aseh Docheh Lo Ta'aseh," such as the Aseh of Milah which overrides the Lo Ta'aseh of cutting Tzara'as from the body (Shabbos 132b). In that case, the confrontation between the two is created by the Mitzvah of Milah itself. The Mitzvah commands, "Cut this skin even though it has Tzara'as." With regard to Matzah, the Mitzvah never commands, "Eat this Tevel.") (A similar answer is suggested by the SHA'AGAS ARYEH (#96) when he discusses the opinion of the BEIS YOSEF OC #11.) <<
Question-
What about Kilayim B Tzitzis? The Torah doesn't say Use Shatnez. It says put on Tzitizis. It doesn't say these tzitzis. In fact it s only if he chooses to wear a 4 cornered beged the 4 corners must have tzitzis placed on them.
If this is considered in opposition than we can also say that the Torah says Eat Matzoh. Eat Matzoh even if it s Tevel.
(b) The KOllel writes further-
>> Finally, it appears that the principle of "Aseh Docheh Lo Ta'aseh" does not apply here, simply because one is not permitted to do a Mitzvah at the expense of someone else! If one eats Tevel in order to fulfill the Mitzvah of Matzah, he steals from the Kohen's portion of Terumos and Ma'aseros that has not been separated from the Tevel. It is obvious that one may not steal someone else's Lulav in order to fulfill the Mitzvah if he has no other Lulav. Similarly, one may not steal the property of Kohanim (Terumos and Ma'aseros) in order to fulfill a Mitzvah. <<
Question-
That would be a chidush that Tevel is momon HaShevet, even though we ordinarily don t look at Tevel as a mixture of Terumah and Chullin but as a cheftah of isur Tevel. A lulav is someone s momon. This would be taking Tevel to another level. Very interesting.
(a) Actually, according to many opinions the Techeiles of Tzitzis must be made out of wool (see Rambam 3:6), hence requiring a person to put woolen Techeiles on a Beged of Pishtan. Especially if Rabeinu David holds that Tzitzis is a Chovas ha'Beged, he would clearly hold that the Torah is telling us to put a woolen string on a four cornered Beged of Pishtan, like the case of Milah you mentioned above (for more answers, see Kesef Nivchar end of Klal 134 DH "Psak").
(b) I agree that this is quite an interesting Chidush. It definitely cannot be said as a blanket statement for all Tevel, as there is still an Isur Tevel (at least an Isur Lav, see Sefer Hachinuch Mitzva 284 and Minchas Chinuch 284:6 who says many say that this is also a Chiyuv Misah) for a Levi who eats grain that did not have Maser Rishon taken, or for a Yisrael who eats grain that did not have Maser Sheini or Maser Ani taken off. This means that this answer would not apply to a Levi or Yisrael who would eat such Matzah, as he would not be stealing, for he, too, can eat the Maser which needs to be taken.
All the best,
Yaakov Montrose