There is no way this is true. The Celtics would need to have the cap room to give George an extension after trading for him. They can do that without Hayward, but cannot possibly do that after signing Hayward.
100% Fake.
I am not a cap master like you are, but if we match salaries and re-sign him at the end of the season we don't have enough room? Also, this guy is a Boston media guy, so I wouldn't classify it as fake, he's hearing it from somewhere, that's why he's reporting it. That doesn't make it true though.
We can't have a 3-year extension in place at the end of next year. That's illegal. And that's not what he's saying. "A 3-year extension in place" means he's going to be traded and given a new contract as part of the trade, (like what we did with KG 10 years ago.)
It's fake. Either this dude is an idiot who passes along whatever he hears without evaluating it, or he made it up himself. I'll lower my fake rating to 99%, because it's doable if we trade Horford, and we'll be charitable and say there's a 1% chance Horford gets traded to make this thing happen. But only 1%.
Otherwise, you could literally trade Tatum, Brown, Smart, Crowder, and Bradley and still not have the cap room to do this. That's how impossible it is. It's not close but difficult. It's simply not happening, because there is no way for it to happen in the CBA.