Shobogenzo Dōtoku XI: Friends in Expressing the Truth.

Shobogenzo Dōtoku concludes with some more discussion of the hut master koan:

Truly, if Seppō and the hut master were not “buddhas alone, together with
buddhas,” it could not be like this. If they were not one buddha and two
buddhas, it could not be like this. If they were not a dragon and a dragon, it
could not be like this.

"Buddhas alone, together with buddhas" is a favourite phrase of Master Dogen's from the Lotus Sutra. It at once suggests the mutuality and Independence of someone in the act of realising buddha in practice/conduct: The person is an independent actor, but his/her conduct is the same as that of all buddhas across time and circumstances. Chapter 91 of Shobogenzo, Yui-butsu-yo-butsu, is dedicated to this phrase.

The black dragon’s pearl is tirelessly guarded by the black dragon, but it rolls
naturally into the hand of a person who knows how to take it.

The black dragon's pearl represents the realised truth. This sentence suggests natural effort without striving for a result.

Let us remember: Seppō testing the hut master, the hut master seeing Seppō,
expression of the truth, nonexpression of the truth, [the hut master] having his
head shaved, and [Seppō] shaving his head. So, in conclusion, there are ways for
good friends in the expression of the truth to pay unexpected visits. And
between friends who are unable to say anything, although they do not expect
[recognition], the means are already present for their selves to be known.

Master Dogen presents Seppō's words as an 'expression' of the truth and the hut master's real act of coming forward to have his head shaved as a 'nonexpression' of the truth. He's not saying that the hut master has not expressed the truth, but, positively phrased, that he has expressed the 'nontruth', the truth of real conduct that is not contained in words and meaning. He connects this to 'shaving' and 'being shaved' suggesting an intimate mutuality and balance between 'expression' and 'nonexpression': words/meaning and direct practice working in a friendly relationship. He indicates also that the means of knowing the self are already present in the absence of words.

When there is learning in practice of knowing the self, there is the reality of
expressing the truth.

Master Dogen concludes by presenting our own learning-in-practice of realising the nature of our self, the words and actions to that non-end, as the real stuff of expressing the truth.

Shōbōgenzō Dōtoku
Written and preached to the assembly at Kannondōrikōshōhōrinji
on the fifth day of the tenth
lunar month in the third year of Ninji.