The Gemarah brings a Baraisa that a Cohen who does Kabollah with his left hand is Pasul. The Gemarah more clearly in Bechorot 45B states that a Left handed Cohen ( a Lefty ) is Pasul for Avodah. The question is why Lefty Cohanim are allowed to Duchin ? since this represents Avodah, they should be Pasul ? Are we to answer , as per O.C. 128:38 MB 139 , that since there is only an Isur, we are more Mekul for Duchining ?
Thanks,
Shmuel Katz
The Taz (O.C. 128:27) states that the Hekesh which equates Avodah and Birkas Kohanim only means that Mumin which a Kohen can easily fix are Pasul for Birkas Kohanim (sitting, being drunk). Otherwise, even Balei Mumin who are in the category of "Bnei Aharon" are Kosher for Birkas Kohanim. [Though one could make an argument that this is b'Yado l'Saken, I think that it is difficult enough to be deemed Ain b'Yado l'Saken (see Taz ibid. at length).]
All the best,
Yaakov Montrose