I don't know that Hahn needs to have an agreement with a third team to make the trade.
As long as Machado isn't significantly hurt, he'll be able to flip Machado basically any time he wants to. Teams will be lined up around the block to do it.
The real question is how much less in value will the prospects Hahn would get in a flip be compared to how much he has to give up. If he figures he'll get, for example, 75% the value of the prospects if he makes the trade later at the deadline, would that diminishment in prospects be worth the trade? If making the trade changes the Sox chances of signing Machado long-term from 1 in 20 to 1 in 5, is that worth it?
The Cubs flipped Jason Hammel midseason 2014 and then re-signed him in 2015, and he logged in a lot of innings for two playoff teams. The few months Hammel was with the Cubs convinced him that it was an up and coming team, and he wanted to be part of the winning. Obviously Machado is in a different stratosphere in terms of quality of player, but the example shows that it's happened, and I think the 2018 Sox are in a comparable place to the 2014 Cubs in terms of their rebuild timetable.
There are so many moving parts to the Sox trading for Machado, that it's really difficult to predict. It could work out that the Sox get more when they trade for Machado than they give up. Hypothetically, they could be in a position where St. Louis, Boston, and the Yankees are all bidding for him, whereas now - if the rumors are true - Baltimore has essentially taken two of those three teams out of the bidding. If that happens, it's a win for the Sox regardless of where Machado signs heading into 2019.