Innocent until proven guilty. He doesn’t have to prove innocence, and he can’t prove a negative no matter how extraordinary he is.
He has admitted he never told his wife about the money given to Ms. White.
Innocent until proven guilty is a standard that’s only used in criminal court. In civil court, Cain would be the plaintiff, and he’d have to prove that the defendant (pick one) was guilty of fabricating the allegation against him, which not-so coincidentally would also affirm his innocence.