Credit Dr. Esther Leibes, the librarian of the Gershon Scholem Collection, whose sharp eye caught this treasure. In 2009, The European University in St. Petersberg, Russia published "the Jewish Nation- Pictures from An-sky's Ethnographic Expeditions" which he photographed over a hundred years ago. The last photograph is of a document, an autograph letter of R. Nahman of Breslev.'Rabbeinu' signs it Nahman of Uman, from where it was, presumably, sent. The heading records the date, Rosh Hodesh Tammuz. If in Uman, this would be 1810, in R. Nahman's final year, only months before his death on the following 18 Tishrei. He writes to his beloved daughter Edel, requesting that she collect all the debts owed to him. In the postscript he asks to be updated on the news from his followers back home. Here's the transcript, by Eliezer Heshin, and his commentary.
It wasn't difficult to authenticate this document. It's visibly similar to the known autograph letters- one to his daughter Sarah, published in תמונות ומסמכים חסידיים Part IV vol. I, NY, 1989, and the other- ms Schocken 70148 (our microfilm film 46860) addressed to his brother Yehiel Zvi.
In the publication, there's not a clue as to where and when An-sky photographed this letter. If anyone knows about it, we would be glad to be informed.