One beat writer from each team has tweeted this deal:
Source: Haren to Cubs from Marlins.— Gordon Wittenmyer (@GDubCub) July 31, 2015
#Marlins sending Dan Haren to the Cubs— clarkspencer (@clarkspencer) July 31, 2015
Haren is 34 and is on the last year of a contract that is apparently going to be paid by the Dodgers (who traded him to the Marlins over the winter), so there's not any financial risk to the Cubs other than a pro-rated portion of the minimum salary.
Haren is 7-7 this year with a 3.42 ERA and 1.093 WHIP in 21 starts covering 129 innings. The numbers look good, except for the 21 home runs he's allowed, kind of a lot. Six of those have been hit in three starts since the All-Star break, perhaps indicating that Haren's velocity is down. Or his location is bad. Or something.
Depending on what's being given up, this is a good, low-risk acquisition. He's got to be better than Clayton Richard or Dallas Beeler, and since someone else is paying his contract, if he's bad, he can just be released.
More info as it comes in. Here is a hint as to what the Cubs are giving up: