Why did Yosef say G-d caused his selling? Bereishis (50:20)

Bereishis (50:16-21): 16. And they sent a messenger to Joseph, saying, Your father did command before he died, saying,17. So shall you say to Joseph, Forgive, I beg you now, the trespass of your brothers, and their sin; for they did to you evil; and now, we beg you, forgive the trespass of the servants of the God of your father. And Joseph wept when they spoke to him.18. And his brothers also went and fell down before his face; and they said, Behold, we are your servants. 19. And Joseph said to them, Fear not; for am I in the place of God? 20. But as for you, you thought evil against me; but God meant it to good, to bring to pass, as it is this day, to save much people alive. 21. Now therefore do not fear; I will nourish you, and your little ones. And he comforted them, and spoke kindly to them

One obvious explanation is that everything is from G‑d (i.e., hashkofa protis – Berachos 33b, Kesubos 30a) and everything is for the good (i.e., Berachos 60b – “R. Huna said in the name of Rab citing R. Meir, and so it was taught in the name of R. Akiba: A man should always accustom himself to say,‘Whatever the All-Merciful does is for good”), and that Yosef was either reminding his brothers of these elementary hashkofa principle or they didn’t know it and thus he was teaching it for the first time. 

But that can’t be because that was not their father Yaakov’s view of hashgacha protis. Berachos (4a) asks why despite G‑d’s promises to Yaakov, he was described in Bereishis 32:8 as being very afraid of Esav. It answers because he was afraid that he had sinned and thus not deserving of G‑d’s promises. Thus Yaakov felt that hashgacha protis providing protection only applied if he were without sin. This is in fact the view of the Rambam (Shemona Perachim Chapter 7) that Yaakov himself was lacking in Bitachon. Similarly Rambam (Morech Nevuchim 3:18) notes that only if there is absolutely complete cleaving to G‑d is a tzadik protected by G-d. The Ramban (Emuna and Bitachon Chapter 1) said that Yaakov didn’t want to rely on bitachon but only wanted what he deserved according to din. In fact the Netziv notes (Bereshis 37:13) that a man can be hurt by the free will of another – even though he doesn’t deserve the suffering. He cites the Zohar in support of this idea.

In sum, Yaakov did not hold that everything is from G-d  - because man can use his free will to cause harm not desired by G‑d and that only a perfect tzadik can count on G-d protection.  Therefore this could not be the message that Yosef was conveying to his brothers.

What he was saying instead was referring to an old dispute he had with his brothers after he reported his dreams which indicated that he was destined to rule them. Yosef was saying that what had happened including his being sent to Egypt was part of a Divine plan for the Jewish people for which he served as the agent. This was not normal hashgacha protis of an individual but rather hashgacha protis of the representative of the Jewish people. He was thus telling them that this was clear proof that the dreams were valid and proof that he was the leader of the Jewish people and thus it was G‑d’s will that he rule over them.