Do we by grace destroy free will? God forbid! We establish free will. For even as the law is not destroyed but established by faith, so free will is not destroyed but established by grace. The law is fulfilled only by a free will. And yet the law brings the knowledge of sin; faith brings the acquisition of grace against sin; grace brings the healing of the soul from the disease of sin; the health of the soul brings freedom of will; free will brings the love of righteousness; and the love of righteousness fulfils the law. Thus the law is not destroyed but established through faith, since faith obtains grace by which the law is fulfilled. Likewise, free will is not destroyed through grace, but is established, since grace cures the will so that righteousness is freely loved. Now all the stages which I have here connected together in their successive links, are each spoken of individually in the sacred Scriptures. The law says: ‘You shall not covet’ (Ex.20:17). Faith says: ‘Heal my soul, for I have sinned against You’ (Ps.41:4). Grace says: ‘See, you have been made well: sin no more, in case a worse thing comes upon you’ (Jn.5:14). Health says: ‘O Lord my God, I cried to You, and You have healed me’ (Ps.30:2). Free will says: ‘I will freely sacrifice to You’ (Ps.54:6). Love of righteousness says: ‘Transgressors told me pleasant tales, but not according to Your law, O Lord’ (Ps. 119:85).
How is it then that miserable human beings dare to be proud, either of their free will, before they are set free, or of their own strength, if they have been set free? They do not observe that in the very mention of free will they pronounce the name of liberty. But ‘where the Spirit of the Lord is, there is liberty’ (2 Cor.3:17). If, therefore, they are the slaves of sin, why do they boast of free will? For ‘by whatever a person is overcome, to that he is delivered as a slave’ (2 Pet.2:19). But if they have been set free, why do they puff themselves up as if it were by their own doing? Why do they boast, as if their freedom were not a gift? Or are they so free that they will not have Him for their Lord Who says to them, ‘Without Me, you can do nothing’ (Jn.15:5), and, ‘If the Son sets you free, you shall be truly free?’ (Jn.8:36).
Augustine, On the Spirit and the Letter, 52
Note: From this quote, we clearly see that Augustine understood "free will" to mean free from the bondage of sin. But to those without the Spirit he asks this rhetorical question shoing he affirms that the unregerate have no true free will: "If, therefore, they are the slaves of sin, why do they boast of free will?"