Trevor Ariza is allegedly their plan B. I think they'd be willing to give him the four year, 48 million dollar contract he reportedly would like to get. Green is going to be a potential trade target for teams that wanted to sign Ariza or Luol Deng and missed.
Here's the James-Ariza-Deng triangle.
If James stays in Miami, Cleveland signs Ariza, and Washington tries to work out a sign-and-trade with Cleveland, wherein they get Deng and Cleveland gets Ariza and a second round pick or two for helping out, since Cleveland can sign Ariza with cap space. If Washington and Cleveland can't work something out, Deng is likely to either go to Houston or Atlanta, with an outside chance of Dallas if Houston matches Parsons offer sheet and Deng hasn't signed anywhere by the (unlikely).
If James goes to Cleveland, Ariza resigns with Washington, and Deng goes to Atlanta, Houston, or Dallas again (see above).
Washington doesn't have the cap space for Green. They hold the Bird rights to Ariza, so that's why they're a destination for Ariza or Deng.
Cleveland would be interested in Green as a plan C, if they're interested at all. There could be some tough optics in sending Boston more picks for Green after sending Boston Zeller and a pick for cap room.
Atlanta seems the most likely jilted Deng suitor, if they are jilted at all. He's definitely their plan A at this point, and in my opinion would fit nicely on that team. It would also let them try again in free agency next year when Green opts out.
Houston has seemed uninterested in Green, and they don't actually have the space to take him on unless they trade Lin or don't match Parson, so he isn't a good fit there.
Dallas might be interested, because they want to do something for Dirk, and will be low on options if Houston matches Parsons.
Accordingly, I think Charlotte also would be in the mix if/when Utah matches Hayward's offer sheet. They have plenty of cap space and no other noteworthy targets. Like Atlanta, they could always try again in free agency next year.