After some research, my impression is that the Scottish and French origins of Osman Ali (Osman Digma) are entirely made up, including the name George Nisbet. It looks like what is now called "fake news". The earliest occurrence of this story I could find is an article that appeared almost simultaneously in several French newpapers in December 1888, for instance in Le Figaro (19 Dec 1888, p. 5, bottom of 3rd and top of 4th column). The article starts with "German newspapers give an extremely romantic biography of this African." I could not find any German article on him of that time (but I may not have looked at the right place).
The German version of his Wikipedia article used to contain the sentence "He was often mistaken for a Frenchman named George Nisbet from Rouen", but it has now been removed. Only the French version still contains an allusion to Rouen, but states that "his birthplace is unknown". Neither is sourced.