This is a great accusation to make, but after reviewing the facts, the conclusion seems indisputable.
|THE PHOTO OF THE LETTER AS IT APPEARED IN AUCTION CATALOG|
In the copy of the letter, most likely copied in the 19th century from התאבקות the letter is complete and nothing is burnt or missing !
Jehoshua Pierce, a dealer who specializes in this field and who has seen numerous Mendelssohn pieces informed me that he independently made the conclusion that the handwriting on the letter offered was not of Mendelssohn. For comparison, see below a letter in the Leo Baeck Collection written by Mendelssohn.
|A MENDELSSOHN LETTER FROM THE LEO BAECK COLLECTION|
In addition, there are differences in the actual handwriting of the one offered for sale and Mendelssohn's known handwriting, as well as several grammar differences from Mendelssohn's own style. For example, Berlin is spelled on the above letter בערלין while Mendelssohn wrote is as ברלין, and see the header of the page which has the acronym בע"ה while Mendelssohn wrote is as ב"ה.
I think we can safely assume that the sale of this letter was done unknowingly by all parties involved, but this case just reinforces what most of us already know, that even auction houses can err. A buyer of anything, especially of something of such historical importance should do his own diligence in evaluating the item offered.
Many Thanks to Yosef Goldman for discovering the original letter in התאבקות and sharing with me the source and pointing out the difference.