What is an explanation for these lines from Act 1, Scene 3: "I neither lend nor borrow/ By taking nor by giving of excess"?
In Act I, Scene 3, Bassanio and Antonio negotiate for a loan of three thousand ducats; this loan is for Bassanio, but because Antonio has all his assets tied up in various ventures, such as his three merchant ships that are still at sea, he must borrow in order to assist his friend. Although Antonio has antipathy for Shylock, he knows that "the Jew's" greed will entice him into loaning the money as he will make a profit since interest will be charged on this loan. Antonio is not worried about repaying this loan because he anticipates profits from his ships which will have sold their cargoes.
When Bassanio introduces Antonio as the supporter of his loan request, Shylock comments in an aside about his feelings of hatred for the Christian merchant who drives Shylock's interest rates down because he does not charge any on his loans. Seeing his hesitation, Bassanio asks, "Shylock, do you hear?" (1.3.48) Shylock replies by saying that he was deliberating about how he would obtain the three thousand ducats to loan Bassanio. Then, he acknowledges Antonio by telling him he and Bassanio were just discussing him.
Antonio explains that although he never deals in usury--
Shylock, albeit I neither lend nor borrow
By taking nor by giving of excess....
--like Shylock, yet in order to help his friend, he will make an exception about the paying of interest. Of course, Antonio has no idea of what Shylock will propose as the "excess."