Update 4:00pm EDT:  The Donald Trump campaign has declined the Bernie Sanders invitation to debate.  “it would be inappropriate“:

(link)
Unfortunately (because many of us wanted the alternate), the smart play argument won out.
By candidate Donald Trump initially entertaining the Sanders invite the media storm elevated Sanders California profile. As a consequence, by declining the debate Trump initiates a sympathetic energy release amid the electorate toward Bernie which will, most assuredly, ensure Sanders wins the California primary against Clinton.
The long game of Clinton’s nominee marginalization is better served with Sanders winning the CA primary.  A well played maneuver of strategic political chess.

EARLIER –> Sen. Bernie Sanders’ debate challenge to Donald Trump is still a topic of discussion for both candidates, leading to speculation that it could actually happen.
…AND, more importantly, it appears the DNC party apparatus is beginning to realize how devastating Bernie Sanders initial proposal for a debate with Donald Trump might be… AND,… and now a tech company, Traction and Scale has stated they are willing to put up the first $10 Million just for the live-streaming aspect:

“Yesterday, Mr Trump asked for a $10 [million] donation to charity in order to accept Mr Sanders’ challenge to debate him.  We are willing to offer that $10 million donation in return for the opportunity to host the debate” – Richie Hecker, chairman and CEO of Traction and Scale

According to Buzfeed News Mr. Richie Hecker would anticipate additional mainstream broadcast media would also be interested:

We would host the debate as a physical event and live stream it to the world. As a technology incubator and investment company, Traction and Scale builds transformative user experiences and will host the debate on our cutting-edge mobile technology. We have distribution to 250MM Americans on social media through this channel.
The debate format would focus on compromise and solutions. We would invite the candidates to look at the forum less as a debate, and more as a negotiation for the future of America. We believe that Mr Sanders and Mr Trump collectively represent the voice of the American people. We are confident that convening the voice of the people in a nonpartisan forum will spark the revolution and make America great again.

One of the bigger hurdles appears to have been overcome as ABC News now reports their contacts with the Democratic National Committee and the Republican National Committee have said they don’t believe there’s anything that would prevent Trump and Sanders from debating, if they agree to do so.
The DNC is obviously stuck between a rock and a hard-base.
Hillary Clinton, the preferred candidate of the DNC, has refused to debate Bernie Sanders after initially agreeing to do so prior to the California primary.   If the DNC moved to “officially” block Sanders from participating – there would be EPIC PANDEMONIUM amid the party base.
However, the DNC also knows such a historic and unprecedented debate would significantly diminish the candidacy of Hillary Clinton who would effectively be cast aside into the land of political irrelevance precisely at the same time her campaign is struggling with a lack of enthusiasm and support.
When you combine the recently released State Dept. Inspector General report on Hillary’s email fiasco, and the growing number of mainstream democrats in the media now openly admitting Clinton lied about the entire construct, with the lack of support for Clinton amid an increasingly frustrated base – well, you can see the obvious political damage that immediately transfers to Mrs. Clinton.
Her campaign could quite possibly implode.
In an effort to protect Hillary Clinton, today some mainstream Democrats began attacking Bernie Sanders for initially proposing the debate with Trump:

[Via CBS] Sen. Joe Manchin, D-West Virginia, told Politico Thursday that the possibility of a debate was “bullshit.”
“That confirms what we’ve been saying,” Manchin told the news site. “Why would you expect Bernie should be considerate or be nice or be working to bring everyone together? Why? He’s not a Democrat.”
California Sen. Dianne Feinstein, whose state holds its primary on June 7, posited that the debate was a publicity ploy.
“What he’s trying to do,” Feinstein told Politico, “is bring attention to himself.”
Michigan Sen. Gary Peters also told the news outlet, “I don’t know why he would do that.”
“I think it’s time to start to winding down the primary,” Peters said. “It’s time to move on.”
The naysaying members of the Senate’s Democratic caucus underscore the increasing tensions between the party and Sanders, whose sometimes vicious primary battle against front-runner Hillary Clinton has led to some division in the ranks. (link)

CBS is trying to play it down.  Cue Major Garrett (this was prior to the $10 million being put up by the California tech company):