Sforno on Genesis 15:3/ספורנו על בראשית ט״ו:ג׳

1בן ביתי יורש אותי, even though, in the end, You will give me biological offspring, as You have said in 12,7 לזרעך אתן את הארץ הזאת, “I will give this land to your descendants,” such children will only be minors at the time, so that my servant Eliezer will have to raise them and guide them, in effect being my heir.