Of course it never happened (and the Clinton campaign had nothing to do with it, either):

One of Sen. Hillary Clinton‘s top financial supporters offered $1 million to the Young Democrats of America during a phone conversation in which he also pressed for the organization’s two uncommitted superdelegates to endorse the New York Democrat, a high-ranking official with YDA told The Huffington Post.

Haim Saban, the billionaire entertainment magnate and longtime Clinton supporter, denied the allegation. But four independent sources said that just before the North Carolina and Indiana primaries, Saban called YDA President David Hardt and offered what was perceived as a lucrative proposal: $1 million would be made available for the group if Hardt and the organization’s other uncommitted superdelegate backed Clinton.
[...]
Members of the Young Democrats agonized about the potential fallout of Saban’s call; his financial offer represented one-third of the group’s 2008 budget. Democratic officials and fundraisers were consulted about how to respond, and at times the discussions were “emotional,” one participant said. “It is scary for them, Haim is very powerful, he has great influence over donors who give to them.”

Another source said that Hardt and others were acutely aware of Saban’s status within Democratic circles and were concerned that their organization would suffer long-term harm if they declined his offer or if news of the proposal became public.

I don’t know about you, but half a million for a superdelegate sounds ridiculously low to me. I’m sure an enterprising superdelegate could do much better putting up her vote for sale on ebay. And just in case you’re wondering, it’s apparently not illegal either:

Those familiar with campaign finance rules say that monetary promises for superdelegates are “problematic,” but may not, in the end, be legally out of bounds.

Personally, I wouldn’t have thought that buying delegates was a campaign finance issue but what do I know? More importantly, would the Hillary we know and love really shrink from anything that isn’t clearly illegal?