I think the price is too steep. Especially when you figure in that a trade would greatly help the Celtics out. Jackson is a better fit, is cheaper, and it most likely gives them another lottery ball to get Luka Doncic next year.
And I'm not sure that Fultz will be better than Lonzo. I think I'd take Ball or Jackson and hope to have two lottery picks next year in order to get Luka Doncic.
This trade will give the Celtics either a top FA (Hayward) and a trade for Butler or a top FA, Josh Jackson, and a likely lottery pick next year (to go along with their Nets pick next year, which is also likely to be pretty good).
The Sixers would already be doing them a favor. I'm not sure I do the deal for next years first and have no idea why a future first is even in the discussions. Well, Colangelo is doing the negotiation, so maybe that explains it.