"John Calvin's letter on usury of 1545 made it clear 
that when Christ said 'lend hoping for nothing in 
return,' He meant that we should help the poor freely. 
Following the rule of equity, we should judge people 
by their circumstances, not by legal definitions. 
Humanist that he was, Calvin knew there were two 
Hebrew words translated as 'usury.' One, neshek, 
meant 'to bite'; the other, tarbit, meant 'to take 
legitimate increase.' Based on these distinctions, 
Calvin argued that only 'biting' loans were forbidden. 
Thus, one could lend at interest to business people 
who would make a profit using the money. To the 
working poor one could lend without interest, but 
expect the loan to be repaid. To the impoverished one 
should give without expecting repayment." 
http://eh.net/encyclopedia/article/jones.usury