Pashut: We KNOW the Brothers did it!
But do we really know that for sure? Read Vayeishev 37:28 thru 36 and show us specifically where the brothers did the dirty deed? Also we notice that Reuven [38:29] was clueless about the sale - which seems a bit strange!
Shloymie: Wait a minute, Rabbi! Rashi says so explicitly - that "vayimsh'chu" refers to the brothers!
RRW: Indeed he does. But simple grammar shows no such antecedent!
Shloymie: we also know from Parshat Vaigash 45:4 that Yosef states that the brothers sold him.
RRW: True but what about Binyamin?
Shloymie: AND in the 10 Harugei Malchut it's obvious that the brothers did it!
RRW: Shloymie you must be agreeing with your namesake Rashi!
But I'm not quite "Rashidox" about this! :-)
The simple p'shat is that the Midyanim sold Yosef to the Yishma'eilim - intercepting Yosef while Reuven was on his way!
Shloymie: but what about vayigash and 10 harugei Malchut?!
RRW: What about the simple Read and Rashbam who suggests this is the P'shat - allbeit not so forthrightly? :-)
RRW: OK OK I see WHY Rashi pinned this on the brothers - But probably based upon Vayigash, not here.
Shloymie: And so what's wrong with that?
RRW: Nothing - but I can read it simply here and answer Vayigash etc.
Re: Vayigash - Yosef simply accused them of the sale because -
A. They expressed that they sought to do that dirty deed
B. That deed was l'maaseh done - albeit via Midyanim
So aisi HKBH is "mitztareif machshavah ra'ah l'maaseh" when it actually DOES happen, even when it's via others.
And so Yosef had a right to accuse them of it, because they were guilty of putting the wheels into motion!
Alternatively, maybe Yosef just THOUGHT that the brothers asked the Midyanim to do it and did not realize that Reuven was clueless and that they beat the brothers to the "punch"
So maybe vaymish'chu is not to be taken so literally that the brothers SOLD Yosef - only that they caused it
Look, I'm not saying Rashi is not WRONG, just that we have a viable "davar acheir" here. One that I prefer.