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Long story short, I think it was related to his Sage Mode. Possibly something that can seal Naruto's Sage Mode permanently robbing him of the ability to use chakra.
What does NB think? Why did Hashi trust this to Sasuke and not Naruto? I think this will be instrumental in how the Sasuke vs. Naruto fight resolves.
I believe this has nothing to do with Naruto. As Hashirama says, "Madara's...taken my Senjutsu Chakra..."
Take a very close look at Hashirama's face on Madara's body:
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Now take another look as Hashirama's face begins to look like/receives the same markings as Hashirama in Sage Mode:
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and so on, etc...
To answer your question - I don't think it will be brought up again.
Hashirama's Sage Mode/Senjutsu Chakra allowed him to heal instantaneously (as a regular person now/no longer edo-tensei) as we see Madara do later on when he gets his butt whooped by the Tailed Beasts along with multiple other powers we didn't see Madara use.
Whatever the actual Binding Technique was and IF Sasuke actually received/accepted it, we later see him miss:
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From Sasuke's responses, I don't think he was just throwing his sword at him plus Madara actually admits he had ample time to do something.
I do however believe if Hashirama did give his remaining Chakra to Sasuke and/or Sasuke accepted it, it was because Hashirama believed if anyone could pull this Binding Technique off it would be Sasuke, and as aforementioned - Madara claims to have given Sasuke a chance to do something.