However, most of the reports seem to be citing a book called Beyond Batman: The Unauthorized True Story of Christian Bale and His Dark Knight Dilemma as the source of the $50 million offer. The book does state that sources close to Christian Bale have expressed their suspicion that Bale now views Batman like Robert Downey, Jr. views Iron Man, hinting that he might be open to returning.
The book also indicates that Christian Bale could make as much as $50 million to return as Batman in a Justice League movie. However, from what we read, the $50 million offer did not come from Warner Bros, but rather it was just a theory of what one of the author's friends thought Christian Bale could make.
Here’s how the book talks about the $50 million offer. “He could probably make $50 million for being in the movie 20 minutes,” my friend observed. “And it would be worth every penny to the studio.”
So yes, the book definitely seems to be suggesting that Christian Bale could return as Batman, but the $50 million offer wasn't something that was actually made to Christian Bale, but was rather someone's opinion.
Obviously this isn't concrete evidence that he has not, or will not, be offered some amount to return, but it seems clear that the offer being widely circulated as coming from credible sources is in fact without merit.