The 'Achtiname' of Muhammad present at the Saint Catherine's monastery in the Sinai Peninsula, Egypt, is not the original charter
but a copy of an alleged
original. Much academic work has been done in discussing its possible authenticity.
There are different sources that mention the tradition associated with it, such as the one mentioned in a well known article by Dr. Aziz Suryal Atiya (The monastery of St. Catherine and the Mount Sinai Expedition). Dr. Atiya was a professor of Medieval History at Farouk I University. A relevant excerpt is cited below:"After the Arab conquest of Egypt in AD 640, it is said that the Prophet Muhammad granted the monks of Mount Sinai a covenant whereby their lives and property became secure under Muslim rule. The existing tradition is that the original charter was taken from the Monastery by Sultan Selim I after the Ottoman conquest of Egypt in 1517. The Sultan, however, gave the monks a copy of it and sanctioned its terms. On the other hand, it is clear from the monumental collection of ancient and modern scrolls preserved in the Monastery Library that the Covenant of the Prophet, whether authentic or forged, was in some way or other renewed" 
Whether this document was a reflection of the sentiments of much later rulers, including some later redactions, or a document contrived by the early Christians to vouchsafe their own protection under Muslim rule will always be questioned by critics. However, the fact that the monastery survives today in tact is a testimony that it had some kind of protection afforded to it throughout its existence under different rulers. It even includes a mosque from the 12th century inside its premises which speaks volumes of religious tolerance during its existence.
Regardless of the sentiments contained within the document or the extent of its authenticity, it nonetheless remains a very interesting script on many fronts.
However, it is not an original, but an alleged copy of an alleged original
likely from the 16th century.
I hope this helps, God willing,
 ATIYA A.S. The Monastery of St. Catherine and the Mount Sinai Expedition, Read April 19, 1951, included in Proceedings of the American Philosophical society Vol 96, No, 5, October 1952, page 578Related Travelogue:SAINT CATHERINE'S MONASTERY, MOUNT SINAI, EGYPThttp://quransmessage.com/travelogues/catherine%20monastery%20FM3.htm