Augustine on arbitrium being liberum or servum

From online source;

"Augustine’s idea of "servum arbitrium" emerges in his response to this heresy: Romans 9:13 is the key text.

While asserting this “enslaved, or servant will,” Augustine also wants to avoid the spectre of an arbitrary or unjust God, and so he asserted that God foreknew Jacob’s merits.

God did not choose Jacob’s future good works, but Jacob’s future faith.

At this time, Augustine held that faith was not a gift, but man’s response to God’s universal vocatio.

Servum arbitrium is also used by Luther, famously.