Li Feng had a rough idea of what Dumbledore wanted.
One possibility was the safe and comfortable Mirror Dimension Professor McGonagall had personally experienced. Another was that Snape had told Dumbledore about Tom's notebook and how Tom had appeared and disappeared intermittently.
Having already learned of the Horcruxes, Dumbledore could not help connecting the notebook to them. Especially since Ginny, a victim who had been controlled by Tom for some time, would certainly receive concern from Dumbledore regarding her mental well-being, as he was the headmaster. He would naturally ask her about what had happened as well.
Dumbledore would most likely learn that the notebook's former owner was Tom—Voldemort during his student days.
Knowing that Dumbledore was dissatisfied with the deal, Li Feng thought for a moment, the corner of his eye twitching as he gritted his teeth. "Don't push it too far. At most, I'll teach the Mirror Dimension to McGonagall too."
As soon as Li Feng finished speaking, Dumbledore, who could tell this was Li Feng's limit, hesitated before smiling. "I have no authority to decide for McGonagall, but I will convey your offer to her."
Hearing that Dumbledore did not continue demanding the notebook, Li Feng breathed a sigh of relief inwardly, though he could not help rolling his eyes. He said disdainfully, "You say you can't decide for McGonagall, but you just want my spells. Wasn't the Mystic Eyes spell I traded to you before satisfactory enough? That thing helped you tremendously."
Dumbledore did not deny the Mystic Eyes' effectiveness. He believed he could find hidden magical objects amid a pile of clutter, but identifying which one among those magical objects was a Horcrux was another matter entirely. So, leaving aside the Mystic Eyes' other effects, its ability to reveal Horcruxes alone was more than enough