Rashi (refer to Shemos 12:40:4:1) and Seder Olam say that Avraham was 70 at the Bris Bein ha'Besarim. Yet it says here that Avraham was 75 when he went to Eretz Yisrael!
Da'as Zekenim: Avraham Avinu went to Eretz Kena'an twice. The first time, he defeated the four kings, followed by the Bris Bein ha'Besarim, when he was 70. He returned to Charan, 1 and left again when he was 75. The Torah is not always in order. 2
Riva (to 12:10); Tosfos to Berachos 7b: Avraham Avinu went to Eretz Kena'an twice. Until verse 15:6 discusses the second time (when he went there to stay), and from 15:7 (Bris Bein ha'Besarim, until the end of Chapter 15), discusses the first time, 3 after which he returned to Charan. 4
Ramban (to Shemos 12:40): We can say that the 430 years are from Yitzchak's birth (and not from the Bris Bein ha'Besarim). HaSh-m said '400 years in a land not theirs', and alluded to additional years - "v'Dor Revi'i Yashuvu Henah Ki Lo Shalem Avon ha'Emori Ad Henah" (15:13, 16).
Moshav Zekenim: Likewise, we find that he was 74 when he defeated the four kings later in this Parshah. (The verses connote that before the war, Lot returned with Avraham from Mitzrayim with great wealth, and chose to live in Sedom, after Avraham was 75! Perhaps also this is out of order - PF.)
HaSh-m said "to the land that I will show you," even though Avraham had already gone there! Perhaps it was to increase his reward; that he was willing to go wherever HaSh-m will tell him. Or, perhaps this was said when he was 70, just the Torah says that he was 75 when he left the second time (PF).
They support this from Berachos 7b. The Gemara says that Avraham was the first to call HaSh-m "Adon," and cites 15:8. It should have cited 15:2! Rather, the Parshiyos are out of order; Bris Bein ha'Besarim took place before verse 15:2. Also, verse 15:5 was said at night, for He asked Avraham to count the stars, and yet in 15:12 it says that the sun set!