What is the ruling if an unknown person is slandered, and the slanderer claims the accused was a slave or a polytheist, but the complainant asserts he is free and Muslim?

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The statement of the complainant is accepted because the default legal status is freedom, and this is the apparent condition, so the claim contradicting it is disregarded, similar to interpreting an explicit slander with something that nullifies it. The premise that Islam is established by one's own assertion does not apply to past states, as a subsequent declaration of Islam does not prove Islam at the time of the slander.