Upon what basis must an oath be taken regarding one's own debt or obligation?

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All oaths concerning an individual's own actions or obligations must be based on definitive knowledge and certainty (Al-Batt wal-Qat'). This applies whether the oath is affirmative or negative regarding what is owed. This view is held by Abu Hanifa, Malik, and Al-Shafi'i. The evidence for this is derived from the hadith concerning the Prophet (peace be upon him) administering an oath to a man, instructing him to say, 'By Allah, besides Whom there is no other deity, he has no right against you.' Furthermore, the hadith narrated by Al-Ash'ath ibn Qays, where the Prophet (peace be upon him) did not object when a man prepared to swear by the form: 'By Allah, he does not know that it is my land that his father usurped from me,' implies certainty is required when affirming or denying one's own actions.