Why is it that in Parashas Pinchas we see the reward of Pinchas (Bamidbar 25:7), even though his to-be-rewarded action was in Parashas Balak (Bamidbar 25:12)? Why wait a Parsha to say what reward he received?
The Beis Yisroel of Ger said this is a lesson. When it comes to קנאות-zealotry,the first thing you do is check things out, what were the motivations what was the intended goals only after it all checks out then comes the reward.The Chasam Sofer adds another point the way to check the legitimacy of the Zealot is to see what happens after he is mocked for his actions.They said about Pinchas בן פוטי his response was not of anger he davened for them to stop the plauge, now you know he is not just the angry type.This is the lesson of the Torah in waiting the Parsha. The Kotzker Rebbe adds another Point why was it after Pinchas proves himself that Yehoshua was chosen as a leader? He answers in a similar vain that while zealotry is good in specific situation as a leader you must understand peoples shortcomings and be able to work with them. Therefore while it was a good in a specific situation it was not what was needed for a leader of the Jews.
The Chasam Sofer citing Tosfos in Yevamos, explains that Pinchas was not appointed to the Kehunah immediately after he killed Zimri, because at that stage, he was unpopular with the people for having just killed a Prince. So Hashem waited until he made peace with them before making the appointment.In the meantime, He gave him a covenant of peace, to enable him to achieve that end. The moment he did, he entered the Kehunah.