The Red Sox can wait it out. Ben Cherington is running the baseball operations department without the formality of having been officially named as Epstein's replacement yet.
The rough deadline would be Tuesday. MLB has a policy prohibiting teams from making major announcements during the World Series, which starts on Wednesday. So if an agreement is not reached by then, the Cubs and Sox would have to wait.
The snag in the talks appears to be two-fold. The Cubs would prefer to pay cash and the Red Sox want something more tangible in the form of players. The quality of the players appears to tied into how many (and which) staff members Epstein can take with him to Chicago.
According to several reports, one from Tony Massarotti, one of the staffers he wants to take is from the medical department. That could well be head athletic trainer Mike Reinold, one of Epstein's closest confidants.
More on this as it becomes available.