What if you go to use your debit card but find you have a $100 spending limit — even if you have more money in your account? Right now, the idea is a bargaining chip being used by some of the nation's biggest banks, including JP Morgan Chase, Bank of America and Citigroup. The change would have a big impact on shoppers. The average family spends $122 on groceries every week, so a simple trip to the supermarket might in the future require a stop at the ATM. It all goes back to new rules that Congress is considering aimed...