I owe a debt of hakaras hatov to a local talmid chacham (I don’t know how he feels about public acknowledgement, so I’ll omit his name) who I asked about applying kitusei michtas shiurei l’kula (see previous post) and was nice enough to get back to me late last night after doing a Bar Ilan search and digging up a mareh makom. It is actually apropos to the parshiyos of these shabbasos. Yevamos 103b has a case of a cloth which is tamei muchlat with tumas tzara’as which is brought into a house, rendering the whole house tamei. The gemara is troubled by this din – since the cloth needs to be burned because of its tumah, why don’t we apply the principle of kitusei michtas shiurei and declare that it lacks the necessary shiur to be metamei the house? The gemara resolves the question by invoking a gezeiras hakasuv, but the hava amina of the question assumes kitusei michtas shiurei works l'kula.
Interestingly, the Minchas Chincuh does not reference this mareh makom in mitzvah #9 when he discusses tashbisu on wheat which is kilayim, but he does mention it in mitzvah #16 (os 19). The footnotes of the new M.C. don’t give you the reference, which just goes to show you that as good as the footnotes are, there is still what to think about.