'Slave girl' in the Bible

For the Scripture says that Abraham had two sons, one by the slave-girl, and one by the free woman.

But the child of the slave-girl was born in the ordinary course of nature, while the child of the free woman was born in fulfilment of the promise.

Yet what does the Scripture say? "Drive the slave-girl and her son away, for the slave-girl's son shall not share the inheritance with the son of the free woman."