Adam Clarke on Genesis 21:10

Cast out this bondwoman and her son - Both Sarah and Abraham have been accused of cruelty in this transaction, because every word reads harsh to us. Cast out; גרש garash signifies not only to thrust out, drive away, and expel, but also to divorce; (see Lev 21:7); and it is in this latter sense the word should be understood here. The child of Abraham by Hagar might be considered as having a right at least to a part of the inheritance; and as it was sufficiently known to Sarah that God had designed that the succession should be established in the line of Isaac, she wished Abraham to divorce Hagar, or to perform some sort of legal act by which Ishmael might be excluded from all claim on the inheritance.