But Mohammed descended from Abrahams union with Hagar and their son Ishmael.. God's promised son is through the union of Abraham and Sarah and their son Isaiah. God gave this promise to Abraham and Sarah.
God never said anything about a "promised son." #1. Unless you mean Isaac.
#2 While it was Isaac to whom the Covenant was promised, which the sign of initiation into is circumcision like all the Israelites had to do in order to belong to the Covenant.
Ishmael was circumcised into said Covenant and blessed with 12 sons, "Princes" and the Ishmaelites are the Arabs today, Mohammed (saws) is a descendant of Ishmael and Abraham, Heber, Shem.
So that tidbit about Isaac is irrelevant as Ishmael was a part of the Covenant himself. Isaac is the typical Biblical second born but favored son, like Jacob for instance. Ishmael is not dishonored though, Esau is. PBWT All.
Hagar(pbwh) shaming seems a little...distasteful, what does it matter that he was only an initiate into the Covenant, everyone but Isaac was? Maybe even Abraham (pbwh) if you want to be super literal was just an initiate into his promised through Sarah (pbwh)miracle child's Covenant with God.