Large banks struck some advantageous deals in the midst of the credit crisis. Now, one appears to be paying off for J.P. Morgan Chase. The deal relates to mortgage repurchases. In the boom, banks frantically made loans and then sold them to bond investors or government-backed agencies like Fannie Mae and Freddie Mac. Now, the buyers are scouring mortgages, particularly defaulted ones, to see if the original lenders failed to meet agreed underwriting standards.