Jefferey,
 
In Pound Era Kenner mentions Mauberley (1919) as the first work to
contain the word "usury," but in "Octave" from Canzoni (1911), EP wrote,
 
Fine songs, fair songs, these golden usuries
Her beauty earns as but just increment,
And they do speak with a most ill intent
Who say they give when they pay debtor's fees
I call him bankrupt in the courts of song
Who hath her gold to eye and pays her not...
 
This is the earliest reference I'm aware of.  Hope it helps.
 
Cheers from the Wet Coast!
 
-Tom McKeown