I don't find any substantiation of this alleged coercion. If there is any it is long into a Google search.
Soros, being a political pariah, is someone that works very very hard to keep his associations under wraps. Knowing that his very association means heavy political damage for those that will do what he would like to see done, he is typically very well insulated from them.
Knowing this it seems that no sooner does the presumptive Republican nominee become clear, his ability to insulate his association with that nominee is 'unfortunately' compromised.
I smell a reincarnation of McCain's illegitimate black baby.
If Soros truly held McCain as an ally, and someone that tracked with his will, he would never let that become public. More likely it serves his purpose to damage McCain and aid Hillary or Obama. All he had to do was give some cash and point the hysterical in the right direction. If he even did that....
McCain takes centrist positions about 15% of the time but he is no leftist.