Bamidbor 25 (7 & 8) records the act of zealotry performed by Pinchas. On possuk 7, Rashi says,
AND PHINEAS SAW — He saw what was being done and he was thereby reminded of the law on this subject (Sanhedrin 82a). He said to Moses, “I have received a tradition from you: he who has intercourse with an Aramean (heathen) woman, zealous people may attack him”. He replied to him: “Let him who reads the letter be the agent for executing it”; — straightway, ויקח רמח בידו HE TOOK A JAVELIN IN HIS HAND, etc. (Sanhedrin 82a).
The Birkat Asher on the Torah points out
ואם בא הקנאי ליטול רשות מבית הדין, אין מורין לו אף על פי שהוא בשעת מעשה וכו' (ראה רמ"א חו"מ תכה, ד)
that the halacha (Choshen Mishpot, Rema 425 (4)) is that if the zealous person comes to ask of the Beth Din whether he may perform his act, we do not so instruct him even at the time of the action.
This creates an apparent problem in the discussion with Moshe recorded by Rashi above. The Birkat Asher has various answers to this question. One is from the Oruch LeNer who says
לפי ה"ערוך לנר" לא בא פינחס להמלך במשה על עצם דין "קנאין פוגעין בו", אלא מסופק היה אם מותר לו לקנא לה' בהיותו כהן That Pinchas did not come to ask Moshe on the halacha in this case but he was in doubt as to whether he could do the act of zealotry, as he was a Kohen and he might become tomei through the action.
I do not understand this answer because at the time that Pinchas did his act, he was not yet a Kohen as Rashi points out Bamidbor 25 (13)
A COVENANT OF AN EVERLASTING PRIESTHOOD — for although the priesthood had already been given to Aaron’s descendants (cf. Exodus 28:40-41), it was given only to Aaron and his sons who had been anointed together with him and to their offspring whom they might beget after they had been anointed. But Pinchas who had been born prior to that and had not been anointed, had not as yet attained the status of priesthood until now. So, too, do we read in Zevachim 101b: Pinchas did not become a priest until he had slain Zimri.
So how can the Oruch LeNer claim that Pinchas came to enquire if he as a Kohen could do the act of zealotry?
related: Question about the actions of Pinchas when he killed Zimri but not about the answer of the Oruch LeNer.