The Haamek Davar explains Genesis 17:16–19 as follows: In verse 16, God tells Abraham that He "will give [Abraham] a son from" Sarah. Because Sarah is no longer physically capable of childbearing, Abraham understands this blessing as meaning that there will be a boy whom Sarah adopts and then presents to Abraham as his. In verse 17, Abraham expresses this understanding, and in verse 18 he prays that the adopted child be Ishmael. In verse 19, God clarifies that "your wife Sarah will give birth to a son for you" and not just rear one. [Note that God never uses the word "born" or "birth" or the like until verse 19.]
However, the wording of the Haamek Davar is puzzling to me. In his commentary on verse 17, he writes that the adopted child
will thereby be called "born to her", like "a son was born to Naomi" [Ruth 4:17]….
Why does Haamek Davar feel a need to explain how the son would be considered born to Sarah, as if this clarifies God's earlier statement that that would take place? God had never said a son would be born to Sarah, that we need to explain such wording as meaning adoption. I mean, the Haamek Davar's wording I just quoted sounds like an allegorical reading of a verse, except that there's no verse he could be explaining this way.
And then in verse 19 he does it again: When God clarifies that Sarah will give birth, Haamek Davar explains that it's
not like you're thinking — rather, she'll give birth, in its usual meaning.
"In its usual meaning" sounds, again, like he's referring to some statement of God's, "give birth", that Abraham had misinterpreted as adoption and that God was now clarifying really does mean giving birth. But there's no such statement! What gives?