Inspired from a quote I used in one of my previous answers here, Patriarch Nikolas Mystikos of Constantinople wrote the following letter to Abbasid Caliph al-Muqtadir:
Two Sovereignties, That of Arabs and of Byzantines, surpass all sovereignties in the world, like the two shining lights in the firmament. For this one reason, if no other, they should be partners and brethren. We ought not, because we are separated in the ways of our lives, our customs and our worship, to be altogether divided nor should we deprive ourselves from communication with one another in default of meeting each other in person. That is the way we ought to think and act, even if no necessity of our affairs compelled us to it.
This is cited in Muslim Perception of other religions by Prof. Jacques Waardenburg, available on google books here.
What sticks out as a sore thumb here is the usage of the word Byzantine. The Patriarch was unlikely to use this word to refer to his country or countrymen as the term Byzantium was used only in the west to refer to ERE, that too only since 1555 after the usage in that sense was introduced by Hieronymus Wolf. Note that Empire had ceased to exist a century before that. In times of the Empire, Byzantium referred only to the Capital city of Constantinople.
It is my understanding that the Byzantine Greeks called themselves Romans (Ῥωμαῖοι - Rhōmaîoi ) or Greeks (Γραικοί - Graikoí).
So from this, it is my assumption that the Patriarch must have used Graikoí or Rhōmaîoi in the original letter. It should also be noted that Abbasids and the Caliphs before them called the Byzantines as Romans (رومی - Rumi).
So my question is, What did the Patriarch call his people in his letter? Greeks or Romans? There must be original Greek text somewhere as Prof. Waardenburg managed to find it as well.