Jeff Green has proven himself well enough that Danny Ainge will always be able to get an expiring contract + probably a first round pick or some young player in return for J.Green.
So there is no need to rush into a trade to get cap flexibility unless you have a clear target who you can sign in free agency OR if that young player / draft pick is so good that you can't pass up on him/it.
Now, Bass or previously with C.Lee ... I don't think they have that type of trade value so if you get an offer like that, you jump at it. But I absolutely believe that J.Green will always be able to get a deal comparable to this one so need to do this without a clear end game (signing Carmelo in FA?) in mind.
I disagree on Bass. For a contender, I definitely think he would be worth a very late first this year. He'd be a real asset to Miami, OKC, Indy, Portland, SA, Houston - top teams that would have a late pick this year.
If Porter had shown mor this year, i think Green and Bass for Porter and thir pick (which would end up out of the lottery, would be starting the point. The expirings are ok but I'd want more back from Washington in this deal since we're seriously improving their team with this deal