Here Augustine discusses the covenant God made with Abraham. Each part of the covenant is full of spiritual meaning and points forward to a blessing that would come ultimate through Christ. And while I might quibble with some of Augustine's specific interpretations, we can't say he isn't careful and thorough in his treatment of the text.
Augustine suggests that Abraham was blameless in the affair with Hagar, since 1) he was honoring his wife's request and 2) he was not pursuing lust, but instead was pursuing children.
I would reply that Abraham may have been honoring his wife, but at the expense of obedience to God.