In fact, you probably did not get the quote from reading the document, but from googling, as the text is quite clear what Augustine is saying here.
...followed by:
As for the location of the quote in question.... after reading through the whole book again I realized it was not there, and upon further searching I realize now I have put the wrong citation. It is, in fact, from a letter, and the correct citation is: August. ad Bonifac. Ep. 106.
So... is the latter comment your version of saying, "Oops, my bad... maybe you actually HAVE the document in your library after all (instead of "probably" Googling it), and my apologies for giving the wrong citation"?
I need to dash, for now; I'll address your other points when I have a free moment, but re: your assertion that St. Augustine thought that Scripture "was the arbiter of disputes" (without any qualifiers to your view), have you read the quote from St. Augustine that I gave
in Comment #2?