Do you doubt that same offer would have been there next summer? If not more?
Around the same, or perhaps even less, due to the fact that he would have 1 year less on his contract. It's the contract you are buying, when you buy a player (playing rights). And how many years that are left on the contract, will affect the end sum.
Juve greens the sale now, because the player (and Raiola) really, really, and I stress, really, wanted the deal done this summer. That much has been clear all along. He wants the Manchester cash and he wants to be part of Mourinho's project immediately. This being the case, Juve sold now while the deal was on the table and the player was injury-free and fit to go. Next year, who knows, he might have been sitting in the infirmary mending a severe injury. There's alot to factor in.
What matters is that he wanted out, and would never have signed a new deal here. In such a scenario, you have to at least listen to offers, and when someone comes in with a transfer record, you'd be quite foolish not to take it.