They can't sign anything that is legally binding until 1 January, but that doesn't mean they can't negotiate via intermediaries and get it all agreed in principle.
@Woland is absolutely correct in his posts above.
It's a breach of football regulations for there to be direct contact between a club and a player before 1 January, but that's not to say it won't have happened. Proving it is the difficult part. We got in hot water over Virgil because we flew him to Blackpool airport to meet Klopp and then they were spotted together at the Pleasure Beach. I imagine Madrid, who are experts in tapping up, won't have been so careless as to pose for photos with Trent outside the Prado.
But I also don't believe they'll have been reckless enough to sign binding legal agreements ahead of time, which means we have until the New Year to turn his head back. And as I said above, we need to start by reminding him how it felt to win stuff with us.