"Curse of the Three Demons" is a short story by Harry Turtledove, first published in Arabesques 2, edited by Susan Shwartz, Avon 1989. It has not been reprinted.
Sa'id ibn Hawqal was a merchant from Damascus. On one trading journey, he travelled deep into the Mongol Empire to the city of Kaifeng. He negotiated an exchange of some of his goods for 2 and 5/6ths ounces of gold with the city's governor Bagagan but was cheated when the governor paid him the agreed weight in silver. Bagagan had two Mongol troopers as guards so all ibn Hawqal could do was angrily curse him in Allah's name. Bagagan had shown only amused contempt but this, in turn, angered him and he curse ibn Hawqal with three Mongol demons.