I am not sure what the rules are for Ma'aser Rishon.
1. Ma'aser Rishon should be given to the Levi. As Kohen is also Levi, does he get Maasar Rishon as well?
2. When Ezra came to Eretz Israel he gave a penalty to the Leviyim. What was this penalty? Did he decide NOT to give Maasaer to the Levy, but instead to give to the Kohen? or to give both.
If this was the case how could he be OKER mitzva deorayta?
I read discussion in Yevamot Perek 85/2 and 86/1.
Reading Beith Habechira LeMeiri, Babba Metzia Daf 11, Amud 1, it looks like there are two opinions.
But reading other sources (Rmabam and others) I understood that he did not give the Levi, but to the Kohen instead.
Aaron Gal, Fair Lawn, New Jersey USA
1. The Gemara in Yevamos on bottom of 86a cites a Beraisa that Rabbi Akiva states that Terumah is given to the Kohen and Ma'aser Rishon is given to the Levi while Rabbi Elazar ben Azaryah states that Ma'aser Rishon is given to a Levi and can even be given to a Kohen. The verse (Bamidbar 18:26) states that Leviyim receive Ma'aser, but R. Elazar learns that since in 24 places Kohanim are called Leviyim, a Kohen can also receive Ma'aser Rishon.
So R. Elazar maintains that since a Kohen is also a "Levi" he, too, can receive Ma'aser Rishon, while R. Akiva maintains that only a Levi can receives Ma'aser Rishon.
2. The Gemara in Kesuvos 26a cites that it is R. Elazar's opinion that one can only give the Ma'aser Rishon to a Kohen. The Gemara asks, where does one find that according to R. Elazar one gives Ma'aser Rishon only to a Kohen and not to a Levi? The Gemara answers that after Ezra's penalty one gives it only to a Kohen and not to a Levi.
(Note that Kesef Mishneh in Hilchos Ma'aser 1:4 writes that Ezra's penalty that one must not give Ma'aser Rishon to a Levi only applied to his times. In future generations, it is permitted to give it to either a Levi or a Kohen. Nevertheless, part of the penalty did remain because before Ezra one could only give it to a Levi and not a Kohen. Thus, the Leviyim lost some of their Ma'aser Rishon.)
3. It seems to me that Ezra's penalty is not considered to be "Oker Davar Min ha'Torah" because one still fulfills the Mitzvah of giving Ma'aser by giving it to a Kohen. According to R. Elazar, who maintains that Kohanim are also called Leviyim, one could give Ma'aser Rishon to a Kohen even before Ezra's time, so even after Ezra imposed his penalty one still fulfills the Mitzvah by giving it to a Kohen.
However, your question may apply according to R. Akiva, who holds that before Ezra one could only give Ma'aser Rishon to the Levi. If so, how could Ezra make us give it to Kohanim?
One can answer this question on the basis of the Gemara (below 90b) that "Shev v'Al Ta'aseh" is not considered to be Oker Mitzvah Min ha'Torah. That is, if Chazal tell us to sit passively and do nothing, this is not considered to be uprooting a Mitzvah from the Torah. Therefore, even if by giving the Ma'aser Rishon to a Kohen one does not fulfill the Mitzvah at all, nevertheless this only constitutes a failure to perform a Mitzvah. It is not an active transgression of a Mitzvah. Chazal have the power to do this.
The Kohen is not transgressing anything, even according to R. Akiva, because Ma'aser Rishon is not Kadosh, so nobody commits a transgression by eating it.
4. If you mean to ask how could Ezra take away from the Leviyim the monetary rights of receiving the Ma'aser Rishon, I can answer with the Gemara (89b) that states that Hefker of Beis Din is a valid Hefker. Ezra's Beis Din had the authority to disregard the monetary claim of the Leviyim to the Ma'aser Rishon.