[Note: To be clear, I am not asking why Ravana wouldn't be harmed or killed by the Sudarshana Chakra; that's obvious. I'm asking why and under what circumstances Vishnu launched the Sudarshana Chakra at Ravana.]
In the Sundara Kanda of the Valmiki Ramayana, Hanuman sneaks into Ravana's palace and sees Ravana sleeping. And in particular he sees Ravana' arms:
That Hanuman also saw the wealthy Ravana's arms tied with golden armlets thrown apart, resembling flag staffs raised in honor of Indra. Those arms had wounds made by stabbing from the pointed edges of tusks of Iraavata and fleshy shoulders smeared with diamond and hit by Vishnu chakra.
My question is, what is the story of Vishnu hitting Ravana with his Sudarshana Chakra (discus weapon)? This excerpt from the Uttara Kanda of the Ramayana describes how when Ravana was attacking Devaloka, Indra asked Vishnu for help in fighting Ravana. But Vishnu refused, saying that Ravana had a boon that he couldn't be killed by any god, but that he would soon incarnate on the Earth as Rama to defeat him:
Do not fear, hear what I have to say. He is invincible by virtue of the boon, and that vicious-souled one is incapable of being vanquished even by the celestials and Asuras. Irrepressible by virtue of his prowess, that Rakshasa, with his son, shall accomplish a mighty object. O lord of celestials, even if I am requested by thee, I shall not withstand the Rakshasa, Ravana in the conflict. Without slaying the enemies in the encounter Vishnu doth never go back — but it is hard to fulfill my desire from Ravana, well protected by the boon. However I do promise before thee, O lord of the Devas, O performer of hundred sacrifices, I shall, soon become the cause of this Rakshasa's death. I shall soon destroy Ravana with his family in due time and afford delight unto the Devas. I speak the truth unto thee, O king of the Devas, O lord of Sachi; do thou divested of fear, fight, O thou gifted with great strength, along with the Devas.
So if Vishnu didn't fight Ravana during the battle of Devaloka, when did he hit Ravana's shoulder with the Sudarshana Chakra?