1)
When did Hashem make a Bris with Avraham?
1.
Radak: "Ba'Yom ha'Hu Karas Hashem Es Avram Bris Leimor l'Zar'acha Nasati
2)
Why does it say "u'Shevu'aso l'Yischak"?
1.
Radak: The Bris was to, i.e. for the sake of, Yitzchak - "Ki v'Yitzchak Yikarei Lecha Zara" (Bereishis 21:12). The Shevu'ah was at Akeidas Yitzchak - "Bi Nishbati" (ibid., 22:16). Hashem told Yitzchak "va'Hakimosi Es ha'Shevu'ah Asher Nishbati l'Avraham Avicha" (ibid., 26:3).
2.
Malbim: This was a second making of the Bris, with Yitzchak. Something repeated twice is like a Shevu'ah (Shevu'os 36a).
