Dick and Jane plot question [Spoilers]

Ok, at the end of the movie McAllister noticed the switched account numbers, foiling Dick and Jane’s plan to divert the money. Then he catches Dick in the bank and basically tells him that he knows what’s up, and writes him a check for $100, and walks off. Dick tells Jane that he “got (McAllister’s) signature”, and she proclaims him a genius. Cue the press conference where they reveal McAllister “donating” 400m to Globodyne’s penion fund.
Huh? How did that happen? The implication is that Dick, Jane, and possibly Frank Bascombe, used McAllister’s signature to somehow carry out their plan of diverting the money and McAllister could have stopped them, but with the press there didn’t want to look like a major asshole. But McAllister had already given the bank his transfer form and knew the account number was wrong, and that Dick and Frank were trying to scam him. He most likely would have been extra careful to make sure his transfer went through correctly. I don’t see how having his signature would have enabled him to be scammed, even with a signature and account numbers you can’t just walk into a bank and vaccuum out someone’s account, especially for an amount that large. And for 400m, it’s kind of surprising McAllister didn’t put up a fight just because he didn’t want to look like a dick in the press. I mean, he was the one who screwed all thise employees in the first place.
So what happened? How’d they do it?