Why did Yosef not buy the Kohanim's fields?
Targum Yonasan: Yosef brought this about out of gratitude for having saved him following the episode with the wife of Potifera. 1
Hadar Zekenim, Riva: When Pharaoh wanted to coronate Yosef, the Egyptians objected - he had adultery with his master's wife! The Kohanim refuted this. Refer to 39:20:3:6.
Hadar Zekenim #2: He benefited them because they were relatives of his father-in-law, Potifera Kohen On. 2
Why does the Torah record that Yosef did not buy the Kohanim's fields?
Tosfos ha'Shalem (3, citing R. Yehudah ha'Chasid): It is lest Yisraelim be stingy about giving Terumah and Ma'aser Rishon, and bringing Ma'aser Sheni to Yerushalayim, which are a fifth in all. Pharaoh did not take the Kohanim's land, and not a fifth from them, due to Avodah Zarah. All the more so you should give a fifth for a Mitzvah, for Hashem gave you Eretz Yisrael for free, and you are Bnei Kel Chai!
Who are these Kohanim?
Rashi: Kohen is one who serves his deity, 1 except when it is an expression of greatness.
Chizkuni: They are officers.
Moshav Zekenim: They are soldiers who ride on horses and in chariots.
Hadar Zekenim: They were relatives of his father-in-law, Potifera Kohen On.
Moshav Zekenim: Pharaoh claimed to be G-d - "Li Ye'ori va'Ani Asisini!" (Yechezkel 29:3). That was a different Pharaoh. According to the opinion that it is the same, we can say that he benefited them in order that they will believe in him and lead others to do so.
