"I have outlived most of my compatriots having to do with the events treated of, and my mind is free from almost every possible prejudice, and in a position where the temptation is strong to obey the maxim, de mortuis nil nisi bonum, so that I trust nothing will be said save where it is necessary to the cause of truth...I am conscious of but one regret for this delay, and that is that in the course of nature it is not probable I shall live so long as to be able to hear all the criticisms, as I am certain many will be made, upon this book, so that I can reply to them, attempting to correct everything that is wrong or mistaken in such criticisms, in justice to those that may be affected by such mistakes, as well as to answer any misstatements hereafter made against the matter of the book, or any attempted contradiction of any fact stated therein, or any new offshoot of calumny against the author." - Butler, preface to his autobiography, 1892 (emphasis added)
Butler died only a year later.
What's silly is to think that this story has credence when there is no mention of it or reference to it between 1865 and 1892.
The story --cannot-- be given historical credibility -because- it cannot be confirmed.
It is really just that simple. All you can say when you present this story is that, "Butler said Lincoln said."
Walt