Or HaChaim on Genesis 17:19/אור החיים על בראשית י״ז:י״ט

1 ויאמר אלוקים אבל שדה אשתך יולדת לך בן. G'd said: "But your wife Sarah will bear a son for you." Why did G'd add the word "but?"

2G'd may have wanted to explain to Abraham that if the gift of a son was something intended only for Abraham, it would suffice to grant his prayer and assist Ishmael to become worthy. However, there was another person to be considered, his wife Sarah. Inasmuch as Sarah was destined to bear a son, he, Abraham, had no right to waive the gift of a son by Sarah in order for Ishmael to grow up worthy of his father. The Torah was very particular using the word לך, "for you," in connection with the son Sarah would bear. She would bear this son in answer to her prayer.

3It is also possible that the Torah here hinted at G'd's displeasure that Abraham was satisfied with Ishmael, someone who contained residual poisonous material from the original serpent. Had it not been for Abraham's prayer regarding the future of Ishmael, his son Isaac might not have originated in what kabbalists know as the סטרא דנוקבא, an emanation on the left and female side of the emanations. Had Isaac originated in the other side of the emanations he might never have fathered an Esau. This matter had ramifications at the time of the עקדה, when Isaac received the soul who would bear children for him, i.e. the moment at which Rebeccah his life-partner was born.