In the conversation with Gilday, Michael references selling the casinos. This could have resulted in a tremendous amount of cash on hand. Plus, as Olivant notes, the Family had myriad other investments presumably accruing in value over decades. So billions does not seem to be unreasonable.

Although I don't know that Michael's commitment of $700M to Immobliere indicates that he was a "multi-billionaire." Immobliere was, to Michael, his one chance to, as Gilday put it, "wash away" the sins of himself and his Family. I wouldn't be surprised if Michael went all-in on that chance. Even if his fortune was "only" $900M, I could see him giving most of it up in that deal. Not to mention that it wasn't a charitable donation: he would have controlled an incredibly profitable business and may have become, again according to Gilday, "one of the richest men in the world."

So, multi-billionaire or not, $700M doesn't seem out of line.