Rashbam on Genesis
על הדבר הזה, concerning the condition stipulated by Avraham.
Sforno on Genesis
וישבע לו על הדבר הזה, he accepted all of Avraham’s conditions as part of his oath.
Or HaChaim on Genesis
וישבע לו על הדבר. He swore to him concerning the matter. An absolute oath concerning a Canaanite girl, and a conditional oath concerning the need to find the girl in Abraham's birthplace should the girl refuse to move to the land of Canaan.