I've never read that Ainge gave Monta a promise. I know that he scouted him heavily, but passed him partially due to him getting injured prior to the draft.
The Greene promise sucked, because his agent wouldn't let the team out of it even with a promise to bring him in as a free agent. At least that's the way it was reported; maybe the agent just said that he wouldn't guarantee Greene would sign with the Celts if he reached free agency.
What I don't understand is, if Danny liked Amir so much, why didn't he buy a 2nd rounder from somebody? A pick in the 50s is probably available for $200k or so. If you really see the potential in somebody, that is chump change.