The Gemara says that we give a choleh the least severe of the forbidden foods available. In the case of tevel, if we are able to be mafrish terumah and there is enough food left, we do. If he needs all of it, there is a machlokes whether we give him tevel or are mafrish the terumah and give it to him
Why can't we just take a tiny drop of food as terumah (since mide'oraisa even a drop of terumah is sufficient,) and the choleh will eat the rest?
If you say that he needs all of it - even that small bit - why doesn't he at least take that bit off and eat it as well? He will only be chayav for chatzi shiur as opposed to giving him tevel where he will have a full shiur and be chayav misa bidei shamayim?
Yisroel PACANOWSKI, Australia
Here is what we answered to a similar question:
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Great question.
Some commentaries suggest that we are dealing with Terumas Maaser, in which case there is indeed a minimal shiur that must be separated [1].
Others suggest that Terumah is a more serious problem because the separation would entail creating an actual forbidden entity [2].
Still others actually reject Rashi's approach in large part because of your Kashya, and instead explain that Rabah means that when you can separate the Tevel into Chulin and Terumah, then everyone agrees that is best, and the Machlokes Tanaim is only when there is no option of being Mafrish [3].
I hope this helps!
Best wishes,
Yishai Rasowsky
Sources:
1. Netziv in Meromei Sadeh https://hebrewbooks.org/pdfpager.aspx?req=40995&st=&pgnum=114&hilite=
2. Aruch ha'Shulchan in the laws of Shabbos https://www.sefaria.org.il/Arukh_HaShulchan%2C_Orach_Chaim.328.10?lang=he&with=all&lang2=he
3. See Parshas Derachim https://hebrewbooks.org/pdfpager.aspx?req=30749&st=&pgnum=155&hilite=