I find the few lines regarding Kohen Onesh Shegiresh hard to understand. If I understand correctly, it seems that the Gemoro is showing that there is a case of Lav HaNitak LaAseh where there is Malkus, i.e. there is a Nitak LaAseh (to remarry the Anusah), and even though the Kohen is prohibited from this, since the Lav is basically Nitak LaAseh, the Gemoro is suprised that the Kohen gets Malkus.
If this is correct, how is the answer of the Gemoro to be explained (Kohanim have extra Kedusha)?
Also, doesn't the Gemoro say elsewhere that really the Kohen could remarry (Aseh doche Lo Saaseh) but there is a Gezera Midrabonon - does this have any relevance!
Gmar Chasima Tova
Mark Bergman, Manchester UK
1. The Gemara understood, as you write, that for the Kohen too the Lav is basically Nitak la'Aseh (see TOSFOS DH HAREI that it should be sufficient that the Lav is partially Nitak la'Aseh) and therefore the Gemara asks why he should receive Malkus if he divorced the Anusah? The Gemara answers that Kohanim are different because the Torah gave them extra Kedushah. RASHI DH H'G writes that since they possess a severe holiness, the Torah was more stringent with them than with Yisrael and therefore they receive Malkus for a Lav ha'Nitak la'Aseh even though a Yisrael does not receive Malkus for the latter. In other words, the Gemara's answer is that because of the extra holiness of the Kohanim they possess a different Din than Yisrael with respect to Malkus also.
2. I am not aware of a Gemara that mid'Oraisa a Kohen could marry his Anusah even though she is a divorcee because of "Aseh Docheh Lo Sa'aseh" but that this is only forbidden mid'Rabanan. On the contrary, MISHNAH KESUBOS 39a states that if the Anusah is forbidden to the man, he is obliged mid'Oraisa to leave her. Gemara there 40a asks in fact why one does not say "Aseh Docheh Lo Sa'aseh" and the Gemara answers that if she would say she does not want to marry him, there would be no Aseh at all - therefore in such a case "Aseh Docheh Lo Sa'aseh" does not apply.
Possibly you were thinking of Gemara Yevamos end 3b that mid'Oraisa one says "Aseh Docheh Lo Sa'aseh" concerning Yibum. See TOSFOS RID KESUBOS 40a (printed on side of Gemara) who explains the difference between Anusah and Yibum.