According to someone who rode an elevator with Nick Young and his friends after the fight between Draymond Green and Tristan Thompson, it was Green who slapped Thompson.  

Reports from the night have been that LeBron and Kevin Durant separated Green and Thompson, after Thompson had slapped or punched Green.  This is an interesting version of events and one that would seem to fit the two men more appropriately.

Via The Sports Daily:

Without video of the actual exchange it’s hard to tell which version is the true one.  It’s also hard to believe in this day and age that a video of the fight hasn’t emerged.  Let’s hope one does.