The just reward
פינחס בן-אלעזר בן-אהרן הכהן השיב את-חמתי מעל בני-ישראל בקנאו את-קנאתי בתוכם וגו’ לכן אמר הנני נתן לו את-בריתי שלום
Pinchas the son of Elazar, the son of Aharon the Kohen, removed My wrath from upon the Jewish people, as he avenged My vengeance amongst them…Therefore, it shall be said that I hereby give him My covenant of Peace
This week’s parsha starts by concluding the episode of the previous parsha. There were many Jews who were involved in lewd behavior with foreign women and idol worship. This had the danger of causing the entire Jewish people to be wiped out in a plague. The grandson of Aharon, Pinchas, volunteered to take action. Although he wasn’t required, he punished the main instigator of the debacle. He stood up, when no one else did. His bold deed gave everyone time to pause, and the sinning stopped. The Jewish people were safe again. Hashem, in this week’s parsha, confirmed that Pinchas behaved properly by taking the law into his own hands. He announced that Pinchas would be rewarded. Chazal make a point of stressing that Pinchas deserved to be rewarded. Why did they feel the need to point this out? The verse seemingly does a fine job of saying that he deserved to be rewarded.
This statement by Chazal can be understood to be addressing a basic question. Hashem told Pinchas that he would be rewarded in this world for fulfilling the mitzvah that he did. How can this be? We have a general rule that there is no reward for mitzvos in this world. The reward earned is saved for the World to Come. Why then was Pinchas rewarded in this world?
There are various explanations given for why there is no reward for mitzvos in this world. One of them is based on the law that it is forbidden for someone to accept payment to give testimony. Obviously, receiving payment would be a conflict of interest, and could compromise the honesty of their testimony. One way to look at our mitzvah observance is that it is a form of testimony. By fulfilling Hashem’s will, we are declaring His existence. That means then that it would be forbidden to receive reward for mitzvos, as that is tantamount to accepting payment for testimony. Only in the next world is there reward.
Nevertheless, the prohibition of accepting payment to give testimony is limited. Only in a case where a person already witnessed something, and then is offered payment to testify to what they saw, is there a prohibition. In such a case, the person is obligated to give testimony anyways, as they saw an event for which their testimony could prove useful. Receiving payment then could compromise their integrity. However, if a person is offered payment to go and witness an event, in order to later give testimony on what they saw, that is permissible. They’re not obligated in the first place to witness it. Essentially, they’re receiving payment to volunteer to become a witness. That is fine.
With this distinction in mind, we can understand now what Chazal are trying to emphasize. Normally, a person shouldn’t receive reward in this world for fulfilling mitzvos. It would be akin to receiving payment to give testimony. The case with Pinchas was different. He deserved to receive reward for his mitzvah, even in this world. Since he wasn’t obligated to take charge, to take the law in his own hands, he was considered a volunteer. Since he volunteered for the mitzvah, it’s similar to someone being paid to volunteer to become a witness. Since that is permissible, Pinchas was able to receive his rightfully deserved reward.
 Based on Divrei Shaul to Numbers 25:11-12, by the author of the Shoel UMeishiv. I found the exact same thing in Toras Ramaz ad. loc. by Rav Moshe Ze’ev Mezuzah. I’m not familiar with the latter, but the two authors seem to be contemporaries as Shoel UMeishiv 3:2:92 responds to one of his inquiries. I wonder who came up with this explanation first
 Numbers loc. cit.
 הלכה ואין מורין כן (Sanhedrin 82a). The Divrei Shaul also proves this from the fact that Zimri would have been allowed to defend himself (ibid). If Pinchas had an obligation to kill him, Zimri wouldn’t have had that permission
 Bamidbar Rabbah 21:1
 Kiddushin 39b
 See Michtav MeEliyahu I p. 4 for a satisfying explanation behind this concept
 The Divrei Shaul and others that came after him quote this idea in the name of the Rashba. After searching through his works, I was unable to find such an explanation by the Rashba. What’s strange is the earliest source I found that quoted this idea in his name was only published in 1825 (Yam HaTalmud Hakdamah § 13, citing his father (who, interestingly, happens to be the Divrei Shaul’s grandfather)). No one earlier seems to quote it. We don’t seem to have it, or maybe it’s a mistaken attribution. Or, quite possibly, I simply missed it
 Bechoros 4:6; Rema to Shulchan Aruch Choshen Mishpat 34:18
 To be honest, I don’t understand why receiving it in the next world isn’t also a problem of receiving payment for testimony
 See Sefer HaMitzvos Aseh § 178
 Rema loc. cit. The source for this distinction is found in Teshuvos HaRashba 3:11
 My impression is קנאים פוגעים בו is even more voluntary than a regular מצוה קיומית. If that’s true, the latter type of mitzvah would still not receive reward in this world