B17 · باب وجوه الاعتراض على العلل الطردية · Objections to a cause that merely goes along with its ruling (ṭard), and when a debater is "cut off" (v2/266–289)

In short

This chapter is a manual for debating a cause that only goes along with its ruling [Overview]. First grant what it proves and show it misses the point, then deny, show wrong construction, or bring a counter-case [B17.01]. Shifting causes because the first failed means being cut off [B17.02].

So for the Ḥanafīs freeing an unbeliever slave discharges the ẓihār expiation, since that text is unqualified [B17.01].

In plain words: How do you answer someone whose cause only goes along with the ruling (ṭard), with no effect shown? Sarakhsī gives four objections, in order. First, grant what the reasoning proves and show it misses the point in dispute. Then deny the feature, its fitness, the ruling, or the tie between them. Then show wrong construction: a cause built to yield the opposite of what its nature yields. Last, a counter-case, which forces him back to the effect.
Example: For the ẓihār (a husband likening his wife to his mother) expiation, the opponent says freeing an unbeliever slave does not count, as in the killing expiation. Granted, not by that feature; but it counts by obeying the command, unless he shows obedience fails here as in the killing expiation, whose text requires a believing slave. Wrong construction: al-Shāfiʿī says one spouse's conversion "is a difference of religion, so separation without a qāḍī, like apostasy". But that difference arises from Islam, which protects the marriage, so conversion alone does not separate them.
Deeper: v1 B17.01; B17.02; B17.03; B17.04

In plain words: When does a debater lose by changing his argument? Sarakhsī allows three shifts. One may move to another cause that proves the first, or, once the first ruling is conceded, to a second ruling by the same cause or by another. But shifting to a new cause for the same ruling means, for most jurists, being cut off (losing the debate). The four signs of being cut off are silence, denying what is seen, denying after conceding, and shifting because the first cause could not prove the ruling.
Example: A debater proves his cause, then adds a clearer proof for the same ruling. He is not cut off, since "the proofs of the Law are lights", like lamp added to lamp. Had he shifted because the first failed, he would be cut off.
Who disagrees: Some allow shifting to a new cause, citing Ibrāhīm's second argument (Q 2:258). Sarakhsī answers that Ibrāhīm moved only after his first proof stood, adding a clearer one.
In his words: "We made this being cut off only where the shift comes from inability to prove the ruling by the first cause": «وَإِنَّمَا جعلنَا هَذَا انْقِطَاعًا فِي مَوضِع يكون الِانْتِقَال للعجز عَن إِثْبَات الحكم بِالْعِلَّةِ الأولى» (v2/288)
Deeper: v1 B17.05