The Butler, Incising is Chapter 139 of the Kuroshitsuji manga.
When the demon tells Ciel that he will claim his soul upon the end of their contract, Ciel objects, saying that he already took his brother's soul. The demon explains that "Ciel"'s soul was only the prize for the summoning—not the prize for the contract.
Ciel is shocked for a moment, but is quickly content with the circumstance that he will die right after getting his revenge as the demon is now his servant and servants always have to be paid. Then, Ciel asks what happens to the souls which are devoured by demons, and the demon answers that they simply disappear. Ciel says that he likes it that they just vanish and adds that he could not pretend to be his brother forever anyway.
He looks at his brother's corpse, which is still lying on the altar, and tells the demon that he needs the blue Phantomhive ring to be able to succeed his father as the earl. However, "Ciel" swallowed the ring right after they were kidnapped. The demon replies that the ring must have already "been expelled from his body," but Ciel is certain that "Ciel" still has it. Then, the demon checks if the ring is still inside "Ciel" and confirms Ciel's hypothesis.
Afterwards, the demon offers to take out the ring, but Ciel insists to do it himself as it is his "duty as eldest son and heir." Hesitating, Ciel takes the knife, with which his brother was killed, and tries to take out the ring—in vain. After his failure, the demon says that retrieving such a small object from inside a body without hitting any bones is an impossible task for children like Ciel and asks again if he can do it. This time, Ciel says yes.
While the demon gets the ring, Ciel realises that, just because you want to do something, it does not mean that you will be able to accomplish it, thinking that it must have been the same for his father and twin brother.
After the demon is done, he hands the ring to Ciel and congratulates him again, addressing him as "Earl Ciel Phantomhive" before correcting himself as he swore to never lie to Ciel. Ciel agrees that he should never address him by his name again, saying that the demon is now his servant and dog.