When Rashi says not Mekach u'Memkar he does not mean that the Halacha is that one cannot do Mekach u'Memkar but rather he is explaining the braisa which is cited as a support for R. Yochanan.

Rashi wants us to know that when the braisa says mischalelin, we should not interpret it to mean not really mischalein, but rather mekach u'memkar. However the braisa is not saying the only way that Sheviis can undergo chilul, and in fact there is an alternative way; namely mekach u'memkar. The important thing about the braisa for R. Yochanan is that mischalelin does work as well.

KOL TUV

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