The 3rd or the 6th-Case-affix may optionally be employed, when the word is joined with another word meaning 'like to or resemblance', excepting tulā and upanā,
Thus तुल्यो देवदत्तेन or देवदत्तस्य, सदृशो देवदत्तेन, or देवदत्तस्य ॥ But with तुला and उपमा the Genitive must necessarily be employed. As तुला देवदत्तस्य नास्ति, उपमा कृष्णस्य न विद्यते ॥
Though the anuvritti of वा was understood in this sutra from the previous sutra, yet the repetition of अन्यतरस्याम् is for the sake of the subsequent sutra. The word च in 2.3.73 attracts the word अन्यतरस्याम् into that sutra; but had the word अन्यतरस्याम् not been used in this sutra, then the word च would have attracted the word तृतीया instead, which is not desired.
On this sutra, Professor Apte says:-- 'Panini says that the words तुला and उपमा can not be used with the Instrumental. But this is against good usage : as तुलां, यदारोहति दंतवाससा (Kum. Sam. V. 24), नभसा तुलां समाररोह (Raghuvamsa VIII. 15). स्फुठोपमं भूतिसितेन शंभुना (Magh I. 4).',