St. Thomas on Justification - Grace and Merit

Merit implies a certain sort of debt: that is, it may be said that something is owed to the man who merits that thing. Is there some way in which it could be said that man merits justification or salvation from God? Aquinas says that there is not:

Grace, inasmuch as it is gratuitously given, excludes the notion of debt. Now debt may be taken in two ways: first, as arising from merit; and this regards the person whose it is to do meritorious works, according to Romans 4:4: "Now to him that worketh, the reward is not reckoned according to grace, but according to debt." The second debt regards the condition of nature. Thus we say it is due to a man to have reason, and whatever else belongs to human nature. Yet in neither way is debt taken to mean that God is under an obligation to His creature, but rather that the creature ought to be subject to God, that the Divine ordination may be fulfilled in it, which is that a certain nature should have certain conditions or properties, and that by doing certain works it should attain to something further. And hence natural endowments are not a debt in the first sense but in the second. But supernatural gifts are due in neither sense. Hence they especially merit the name of grace. [ST I-II, Q111, A1, ad 2; emphasis added]

God does not owe us salvation, and there is no way that he can become indebted to us so as to owe it to us. Consequently, as St. Thomas says here, "supernatural gifts ... especially merit the name of grace."