I'm pretty sure that Danny had anticipated this move from Koby Altman and the Cavs even before he agreed to the trade. It's clear they're trying to get some more assets. They don't want to take Kyrie back. They want this to go ahead, they're getting a great deal as it is.
It wouldn't surprise me either if Danny was expecting and had budgeted to give up a second round pick or some sweetener in anticipation of this even before he agreed to the initial trade, as insurance to make sure the trade didn't fall apart. So he gives up Jabari Bird or Abdel Nader as a good faith thing. Danny's not stupid and I'm sure he would have anticipated this move from the Cavs. There's no way they're fundamentally restructuring this trade at this point.
If Danny gives up anything it will come across as him admitting he wasn't being totally honest about IT's health. For that reason alone I think he stands his ground. Otherwise it's like him saying "Whoops, you got me! Oh well, I tried. Here's another pick to actually make this thing even."
That would really be bad for business in the future when it comes to Ainge trying to make trades. No one would believe him.
Also, by standing his ground, he's showing that what he offered is truly, in his mind, an already great deal for the Cavs. He's proven he's willing to overpay. He's proven that it's ok to deal with the Celtics because you aren't always going to be on the losing end of the deal.
So many people said earlier this summer that "No one wants to trade with Danny because he HAS to win every deal." Well this kills that narrative, if Danny stands firm and doesn't show that maybe he wasn't 100% honest and gives up another asset.
Too many other big picture things going on here. You can't just say "Ah, a second rounder added to this is easy to give up."