Was there a precedent in Islamic history to the abolition of slavery prior to the 19th century C.E.?
Jazak Allahu Khayran
Not according to W.G. Clarence-Smith’s “Islam and the abolition of slavery” or Ehud Toledano’s “Slavery and Abolition in the Ottoman Middle East” but Dr. Muhammad Sa`id al-Buti mentioned that Mehmed the Conqueror in the 15th century called for an “international protocol to be signed by most of the world countries…. [and] managed to convince the Europeans and the other world countries to stop enslaving the prisoners of war. Hence, they signed that charter.”
See: The First Tri-lingual anthology of Imam al-Butis Works (pp. 233-234)
However, no reference to such a momentous charter can be found in the above-mentioned books, nor in the Cambridge History of the Ottoman Empire and Modern Turkey and the Cambridge History of Turkey. I found only a passing mention that, after defeating the powerful Turkoman Shahahshah Uzun Hasan at Otluk Beli in 1473, Sultan Mehmed freed all his own slaves in thanksgiving, totalling 40,000 men and women (Mehmed the Conqueror and His Time by Franz Babinger, p. 316).
Hajj Gibril Haddad