I tried control-f-ing to find the mention of Sāvarṇi but it did not work.
Rig Veda X.62.9,11 speak about Sāvarṇya and Sāvarṇi.
न तम॑श्नोति॒ कश्च॒न दि॒व इ॑व॒ सान्वा॒रभ॑म् । सा॒व॒र्ण्यस्य॒ दक्षि॑णा॒ वि सिन्धु॑रिव पप्रथे ॥९॥
No one succeeds in taking hold of him any more than the back of heaven. The priestly gift of Sāvarṇya spreads out like a river
स॒ह॒स्र॒दा ग्रा॑म॒णीर्मा रि॑ष॒न्मनु॒: सूर्ये॑णास्य॒ यत॑मानैतु॒ दक्षि॑णा । साव॑र्णेर्दे॒वाः प्र ति॑र॒न्त्वायु॒र्यस्मि॒न्नश्रा॑न्ता॒ अस॑नाम॒ वाज॑म् ॥११॥
Let Manu, giver of thousands, leader of the horde, not suffer harm. Let his priestly gift come, aligning itself with the sun. Let the gods extend the lifetime of Sāvarṇi, under whose auspices we, unwearied, won the prize of victory.
Sāṁvaraṇi is found in the Rigveda in one passage, where it naturally seems to be a patronymic (‘descendant of Saṁvaraṇa’) of manu. (Rig Veda VIII.51.1)
यथा॒ मनौ॒ सांव॑रणौ॒ सोम॑मि॒न्द्रापि॑बः सु॒तम् । नीपा॑तिथौ मघव॒न्मेध्या॑तिथौ॒ पुष्टि॑गौ॒ श्रुष्टि॑गौ॒ सचा॑ ॥१॥
Just as at Manu Sāṃvaraṇi’s you drank pressed soma, Indra, and at Nīpātithi’s, at Medhyātithi’s, (do so) at Puṣṭigu’s, at Śruṣṭigu’s, bounteous one.
According to Bloomfield, it is a corruption for sāvarṇi, a reference to Manuऽs birth from the savarṇā, ‘similar’ female who was substituted for Saraṇyū according to the legend.