We learnt earlier that "nicha lei linish lkiyumei mitzvah b'mamonei". Why then is the use of a second biur considered a hefsed - since it is also done in the fulfillment of the mitzvah, can we now apply this principle of "nicha lei linish lkiyumei mitzvah b'mamonei"?
Thanks so much
Marc Abrahams, Beit Shemesh, Israel
To answer your question, we must first explain that in the case the Gemara is discussing, the prohibition not to allow the Terumah to become Tamei is only a rabbinic one. This is evident, because otherwise the Gemara would not be able to permit the prohibition in a case of monetary loss. The reason that here there is no Issur Min ha'Torah is due to the fact that here the Terumah would be lost even without further human intervention. See P'nei Yehoshua 15a.
We may now answer, that there are some rabbinic prohibitions that the Chachamim permitted in the case of monetary loss. (See Shabbos 154b for a further example.) This leniency is due to the fact that rabbinic prohibitions are less stringent and, in some cases, Chaza"l decided that it is possible to be lenient. This holds true even though a person would be happy to spend his money on the Mitzvah had Chaza"l taken a more stringent stance.
As far as Biur Chametz is concerned, see Tosfos on 10b that where one has Chametz one must do Biur even if this will involve monetary loss. This is because in the Mitzvah of Biur Chaza"l were more stringent even though the need for Biur is only rabbinic. Tosfos says that this is because Nicha Lei l'Kiyumi Mitzvah be'Memonei. We must say that if not for this rule, even though Biur is more stringent, Chaza"l would still have been lenient, since the prohibition concerned is only rabbinic.