His statement, “through li'an”: the preposition bi- may indicate causation—that is, on account of li'an—or transitivity, meaning that he averts it by means of such-and-such.
The evidence for this is the saying of Allah, Exalted is He:
“And those who accuse their wives but have no witnesses except themselves, then the testimony of one of them shall be four testimonies by Allah.” [Quran 24:6]
The wisdom in its being four testimonies by Allah is clear, for they take the place of four witnesses. His saying, “by Allah,” indicates that this testimony is coupled with an oath, because he says, “I testify by Allah”—as though he had said, “I testify, swearing by Allah.” This is why Allah, Exalted is He, called it testimony, while the Prophet (peace and blessings be upon him) called it oaths, saying, “Were it not for the oaths, I would have had a matter to settle with her.” (1)
Accordingly, we say that this testimony includes an oath. His saying, “but have no witnesses except themselves”: “themselves” is a substitute for “witnesses,” because the sentence is a complete negation, and the predicate of “have” is “for them.” It is also possible for “have” to be complete in meaning—that is, no witnesses are found for them. Some have said that, in “except themselves,” “except” is an adjective meaning “other than,” as in the saying of Allah, Exalted is He:
“Had there been within them gods other than Allah, they both would have been ruined.” [Quran 21:22]
That is, other than Allah. They said that “except” can carry the meaning of “other than,” but its grammatical inflection is transferred to the word following it. Thus, “except” is an adjective for [omitted in the source text], but its inflection has been transferred to what follows it, where it becomes apparent. This grammatical disagreement does not result in any disagreement regarding the ruling.
The fourth condition is that both spouses be legally accountable; that is, the spouses must be