I think it comes down to the idea that Marley has a personal interest. He feels for Scrooge because he and Scrooge are kindred spirits, and former friends...or associates. And being so close to Scrooge, in that sense, he knows exactly what Scrooge's problem is and how it might be fixed.
He surely wishes to see Scrooge redeemed, in part, because he dreams of himself redeemed but that's not possible any more. It will warm Marley's heart to see it happen.
So I believe Marley undertook this as a personal project, and made a deal, offered whatever was necessary to secure this chance for Scrooge.
As frightening as Marley's apparition is, he's really, I think, a nice guy to go to all this trouble for his friend.
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Friend....gooooood!
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