Yevamos 5a - Mitzvaso bekach

Tosafos on 3b explained that we can't learn aseh docheh lo sa'aseh from the standard case of yibum where we push off eishes ach, bec. that is "mitzvaso bekach". Simply speaking this means that since the mitzvah of yibum only exists in contrast to the issur, the issur could never have been given when up against the mitzvah of yibum. Meaning, mitzvaso bekach would indicate that it must be hutra rather than dechuya.

However, Tosafos 5b is clear that although kelaim in bigdei kehuna is "mitzvaso bekach" and therefore cannot be used as a source to learn from (as Tosafos says by the standard case of yibum), it is not b'geder hutra, rather it is dechuya. Tosafos says that bigdei kehuna is not a weakness in the prohibition of kelaim, bec. we are not matir the issur of kelaim (which would indicate its weakness), rather we push it off b'geder "dechiya" as we would in any aseh docheh lo sa'aseh. Clearly Tosafos holds that "mitzvaso bekach" does not force us to understand that it is hutra rather than dechuya. Even though kelaim bb'igdei kehuna is mitzvaso bekach, it follows the regular format of dechiya i.e. aseh of avodah is docheh the lo sa'aseh.