[The ruling on administering an oath by disavowal from Allah]

Answer – and Allah is the One who grants success: According to the madhhab, it is not permissible to intensify (an oath) by means of a word of disbelief and disavowal from Allah or from Islam. Thus it is (stated) in the commentary and its marginal notes.
And in the marginal notes: It is narrated from ʿAlī (Peace be upon him) and some of the early scholars that it is permissible, and it is narrated from al-Muʾayyad Billāh (Peace be upon him) that he administered such an oath as part of an oath which he strengthened upon someone whom he put on oath, by saying: “If you intended other than this, then you are disavowed of Allah, and ḥajj is binding upon you.” Likewise Yaḥyā ibn ʿAbd Allāh (Peace be upon him) made az-Zubayrī swear the well-known oath, which was that he said to him: “Say: I have disavowed the might and power of Allah, and I have clung to my own might and power; in arrogance toward Allah and deeming myself independent of Him, I did such-and-such.” When Yaḥyā ibn ʿAbd Allāh (Peace be upon him) made him swear this oath, he was swiftly seized (by punishment) – it is said, within two days or three – and he was afflicted with leprosy and died; and he has a long story. End of quotation from the marginal notes on al-Azhār.
I say: What supports the permissibility is the saying of Allah, Exalted is He: “Say, ˹O Prophet,˺ “If the Most Compassionate ˹really˺ had offspring, I would be the first worshipper.” [Az-Zukhruf:81]– on the view that “in” is a conditional particle, and that what is joined to the faʾ is the answer to the condition; and this is what appears to be the case.
Yes, the permissibility of administering an oath by this formula ought to be restricted, namely, that the one being put on oath is an oppressor, as in the story of Imām Yaḥyā ibn ʿAbd Allāh (Peace be upon him) and az-Zubayrī, and as is narrated from ʿAlī (Peace be upon him): “If you wish that the oppressor be hastened with punishment, then make him swear…” and he mentioned this oath, or as he said; or where the dispute and discord will not cease except by means of this oath.
This being so, what appears to me is that the one who swears by it neither becomes a disbeliever nor a flagrant sinner, whether he breaks (the oath) or does not break it, for such a judgment is only given on the basis of decisive proof. This is also because of His saying, Exalted is He: “but those who embrace disbelief wholeheartedly—they will be condemned by Allah and suffer a tremendous punishment.” [An-Naḥl:106] – except if he swears regarding a matter while knowing the contrary, doing so deliberately.
This being said, as for administering such an oath in financial rights and what is connected to them, it is not fitting, due to His saying, Exalted is He, in the context of testimony and oaths: “…“By Allah! Our testimony is truer than theirs. We have not transgressed. Otherwise, we would surely be wrongdoers.” [Al-Māʾidah:107] and His saying, Exalted is He, concerning the liʿān-oath: “four testimonies [swearing] by Allah that indeed, he is of the truthful.” [An-Nūr:6].

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2