1) Tosfos (Berachos 7b, DH Lo) writes that Avraham was 70 years old at Bris Bein ha'Besarim, while at the beginning of Parshas Lech Lecha (Bereshis 12:4) the Torah states that he was 75 when he left Charan.

2) The Rosh in Yevamos 6:12 explains more about when Bris Bein ha'Besarim happened, but we will see that it is not so simple to say that it happened before Lech Lecha.

a) The discussion starts with the Mishnah in Yevamos 64a, that if a man was married for 10 years but no children were born, he may not wait any more.

b) A source for this is that Avraham married Hagar after he had lived 10 years in the Land of Canaan without children.

c) The Rosh writes that Hash-m commanded Avraham, "Go out from your land," but Avraham was "lazy" and did not go. Even after he did come to Eretz Yisrael, he returned to his original place.

d) The proof for this is the fact that Avraham was 70 years old at Bris Bein ha'Besarim. We know this because Bereshis 15:13 states that Avraham's descendants will be afflicted in a foreign land for 400 years. But Shemos 12:41 tells us that they left after 430 years.

e) The way to resolve this contradiction is to say that the 400 years of the Egyptian exile started when Yitzchak was born, while the 430 years started at Bris Bein ha'Besarim. This means that there were 30 years between Bris Bein ha'Besarim and the birth of Yitzchak. Since we know that Avraham was 100 when Yitzchak was born, this means he was 70 at Bris Bein ha'Besarim.

f) Bereshis 12:4 tells us that Avraham was 75 when he left Charan. Bereshis 15:18 tells us that Hash-m said to Avraham in Bris Bein ha'Besarim, "I have given this land to your offspring," which proves that Avraham was in Eretz Yisrael at the time of Bris Bein ha'Besarim. We conclude that he went to Eretz Yisrael before he was 75, so when he went to Charan this was not the first time he had been to Eretz Yisrael.

g) So it is not quite true to say that Bris Bein ha'Besarim happened before Lech Lecha. It is true to say that it happened before he left for Charan but that is not the very beginning of Parshas Lech Lecha. The very beginning of Parshas Lech Lecha is when he went to Eretz Yisrael the first time which was before Bris Bein ha'Besarim.

Dovid Bloom