ka is substituted for kim before a vibhakti affix.,
As कः, कौ, के ॥ The substitution takes place even when the augment अकच् is added. Therefore, the substitute is here क and not अ which latter would have been sufficient for किम् ॥ For म् of किम् being replaced by अ 7.2.102, the इ would be left, which would be replaced by अ, had the sutra been किमोऽत् and the forms would have been the same (क् + अ + अ = क 6.1.97. See 5.3.1, 5.3.13 &c.,