Cercado sui dizionari monolingua (Oxford, Longman) non ho trovato il lemma *usance*. Digotando *usance* su http://www.wordreference.com compare la seguente definizione "the period of time permitted by commercial usage for the payment of a bill of exchange (especially a foreign bill of exchange)".
Sul Sansoni e su www.hoepli.it la definizione risulta essere "scadenza, termine".
Su http://bewley.virtualave.net/credit.html ho trovato " The bill of exchange was a binding promise to pay a specific person a certain sum at some future but proximate date in another town, and hence a change into another currency. This in itself involved a loan, for even if a bill was payable 'on sight', it still took time to be carried from one place to another before it could be paid. It was also agreed that for large sums the person required to pay should have time to raise that sum. This delay, known as 'usance', might be one, two or three months from the day the bill was presented - so the merchant had a credit advance of one to three months. Fair enough, but the lenders had fixed rates of usance, or interest as we would call it today, and also received additional profit on the loan by playing the exchange market. Some merchants would even send bills to themselves to gain credit, bills secured by absolutely no cash at all!"