Peter wrote:I wonder if Sariputta knew the man's potential,
I don't think so.
Soon after the venerable Sāriputta had left, the brahmin Dhanañjānī died and reappeared in the Brahmā-world.
Then the Blessed One addressed the bhikkhus thus: “Bhikkhus, Sāriputta, having established the brahmin Dhanañjānī in the inferior Brahmā-world, rose from his seat and departed while there was still more to be done.”
Then the venerable Sāriputta went to the Blessed One, and after paying homage to him, he sat down at one side and said: “Venerable sir, the brahmin Dhanañjānī is afflicted, suffering, and gravely ill; he pays homage with his head at the Blessed One’s feet.”
“Sāriputta, having established the brahmin Dhanañjānī in the inferior Brahmā-world, why did you rise from your seat and leave while there was still more to be done?”
“Venerable sir, I thought thus: ‘These Brahmins are devoted to the Brahmā-world. Suppose I show the brahmin Dhanañjānī the path to the company of Brahmā.’”
“Sāriputta, the brahmin Dhanañjānī has died and has reappeared in the Brahmā-world.”
(Dhanañjānī Sutta, MN. 97)
From: Bhikkhu Bodhi's notes:
Sati uttarikaraṇīye ("while there was still more to be done"). Ven. Sāriputta had left without giving him a teaching that would have enabled him to arrive at the supramundane path and become fixed in destination for enlightenment. Compared to this even rebirth in the Brahmā-world is described as “inferior” (hīna). This remark has the force of a gentle reproach. The Buddha must have seen that Dhanañjānī had the potential to attain the supramundane path, since elsewhere (e.g., MN 99.24-27) he himself teaches only the way to the Brahma-world when that potential is lacking in his listener.
This episode is briefly alluded to in the Peṭakopadesa (Peṭ. 79; Piṭaka Disclosure 102-3), an early treatise on hermeneutics. It crops up in a discussion of which sayings of the Buddha's disciples should be accepted as Dhamma. Here the Buddha's words to Sāriputta are treated as a "non-congratulation", rather than a rebuke:
But there is also the kind of hearer (sāvaka) who knows the Ten Powered One's province, either limited or unlimitedly, yet he does not know that power [itself] at all beyond the hearing [of it]. As, for instance, in the case of the brahmin exhorted by the venerable Sāriputta. Now that venerable one lacked [the Tathāgata power of] knowledge of variety in faculties and powers (indriya-bala-vematta-ñāṇa), hence by his not knowing the encompassing of other persons, though the brahmin had more he could still have done [i.e. by attaining the noble path], he was instead made to reappear [after death] in the Brahmā-world, and so the venerable [Sāriputta] was not congratulated by the Blessed One.
The treatise then goes on to cite the Kassapagotta Sutta (SN. i. 199) as an example of a disciple (Mahākassapa) whose lack of the Tathāgata powers results in him giving his nephew a teaching that is too high for his capacity, with a similarly unfruitful outcome.
or could have known if he thought to investigate.
There is a commentarial telling of the story in which Sāriputta does
investigate as best he can, but unfortunately his best isn't good enough. I can't locate it at the moment, but it goes something like this: Sāriputta surveys the previous hundred thousand lives of the brahmin and in none of them can he detect any past action that could be a cause for awakening in the present life. And so he teaches him the way to Brahmā instead. But the Buddha —whose psychic powers can go further back than Sāriputta's— sees that in the brahmin's hundred thouand and first
previous life he had heard the Dhamma taught by some past Buddha and in that hearing there was a sufficient cause for stream-entry in the present life. Except that it was too late.