Walmart, which has been making a marketing push to provide financial services to low-income consumers, says it is introducing a first: A cash-back incentive for its prepaid debit card. When customers use the Walmart MoneyCard to buy gas, they'll get 1% cash back on their gas purchases through the end of this year. The Walmart MoneyCard, which costs $3 to purchase, is one of the major products Walmart markets to the "unbanked," along with check cashing, bill paying and money transfers.--Sarah Mahoney