Ibn Ezra on the Torah

1 Place your hand underneath my thigh. There are those who say this alludes to the Brit Mila. However, if this were the case, Abraham would have sworn on his brit mila and not in God's name. But the explanation that I think is the closest [to understanding this custom] is that it was the law in those days for a man to place his hand underneath the thigh of someone in whose domain/service he was in; and the reason was because, "if you are in my domain/service, place your hand underneath my thigh." The master would then sit with his thigh on the servant's hand underneath in accordance with this reason: "My hand is in your domain to do your will." And this is the still the law in the Land of Hodu [India].

1. And the reason for telling us the God of the Heavens and the God of the land is because in this place when a daughter is set up with someone (a shidduch) it is really from Heaven. And this is a secret.

"Thou shalt not bring my son back thither." He must remain in the land of Israel. Abraham said, "you shall not bring my son back" because he (Abraham) was the root.