Woodward has always come up with unverifiable claims that many do not believe.
The claim was published in Shadow: Five Presidents Under the Shadow of Watergate in a chapter on Gerald Ford's presidency written after an interview with Gerald Ford. To me, that makes Woodward's claim credible.
Haig obviously doesn't feel that there was an offer made, Ford did. Whether you believe Gerald Ford or Alexander Haig is up to you, but I personally find President Ford more credible.
I guess you do not believe he interviewed the ex-director of the CIA on his death bed and in a coma? No doctors saw him, no nurses, Casey's family did not see him, nobody saw him and Casey was in a coma but he still says he interviewed him.
The man lies.
Bob forgot that the trick is to wait until the principals are dead before you quote them.