My mother owns a property and has few debts and the property value exceeds the sum of debts but she isn’t able to sell off the property because she isn’t able to find any seller. So does she have to pay khums from when she owned the property or when she sells off the property and clear the debts?

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If she bought the property to live in, and she lives in it, then she do not need to pay khoms
Otherwise, yes.