Commento su Genesi 15:4

AND, BEHOLD, THE WORD OF THE ETERNAL CAME UNTO HIM, SAYING: THIS MAN SHALL NOT BE THINE HEIR. Since Abraham had his son who would be his heir after his old age, the Eternal assured him only concerning the inheritance,²³⁷Ramban here suggests that Abraham’s faith in the first promise (see above, Note 223) remained unshaken. However, as he grew older, he feared that if his son will be born near the time of his death, Eliezer will do with the child as he pleases, and he instead will become his heir. On this matter of inheritance G-d now assured Abraham that he should not worry for his seed will inherit him. (Tur.) i.e., that he should not worry, and his seed will inherit it.
The meaning of the expression, And, behold, the word of the Eternal came unto him, is that while Abraham was still saying, And, lo, my household slave will be mine heir, the word of G-d suddenly came to him, saying This man shall not be thine heir.

לא יירשך זה כי אם אשר יצא ממעיך, for the son would succeed taking over Avram’s leadership role amongst large number of followers while his father was still alive. This is the true meaning of Genesis 25, 5-6 ויתן אברהם את כל אשר לו ליצחק בנו.... בעודנו חי,” Avraham handed over all that was his to his son Yitzchok while he was still alive.” This is followed by the statement that Avraham sent away the sons of the concubines from the presence of his son Yitzchok.

'והנה דבר ה, now G’d’s word in response to his concern about who would inherit him, saying: לא יירשך זה, this loyal servant of yours will not inherit you, כי אם אשר יצא ממעיך, do not err in your understanding of the meaning of the words לך ולזרעך, “to you and to your descendants,” I only referred to your biological; offspring. The word מעיים refers to all the organs within the body, in this instance referring to the reproductive organ. The word is used in a similar sense in Isaiah 48,19 וצאצאי מעיך, “your offspring.”