Is an oath sworn by an unbeliever valid?

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The oath sworn by a non-believer is valid. If the person breaks the oath (hanith), the expiation becomes obligatory, whether the breaking occurred during their state of disbelief or after their conversion to Islam. This view is held by Al-Shafi'i, Abu Thawr, and Ibn Al-Mundhir specifically regarding the expiation due after conversion. The evidence for this is that when 'Umar (may Allah be pleased with him) vowed during the time of Jahiliyyah (pre-Islamic ignorance) to perform I'tikaf (seclusion) in the Sacred Mosque, the Prophet (peace be upon him) commanded him to fulfill his vow. Furthermore, the unbeliever is considered liable for certain obligations, evidenced by the verse stating, "...then let them both swear by Allah" (Quran 5:106). The notion that they are not accountable is rejected, as their exemption from religious duties applies only after embracing Islam because Islam nullifies previous obligations, but obligations incurred via their own vow or oath should remain binding upon them as they originate from their own declaration.