Stock index futures dipped on Tuesday as investors reassessed a vow by China for more flexibility on its currency and turned cautious ahead of data expected to show a slight rise in U.S. home sales.

The Federal Reserve's rate-setting committee also begins its two-day meeting, with a statement to be released on Wednesday. Investors will watch for any minor changes in language that could illustrate the central bank's outlook on the economy.

Optimism over China's weekend decision to give the yuan more flexibility faded late Monday and weighed on world stocks overnight amid doubts about the speed and magnitude of Beijing's intentions.

A host of questions are arising -- the costs factors involved for both consumers and exporters, and was it purely political? said Peter Cardillo, chief market economist at Avalon Partners in New York.

Shares of Walgreen Co fell 0.5 percent to $30.00 in light premarket trade after the company reported weaker-than-expected profit.

U.S. existing home sales for May are due at 10 a.m. (1400 GMT), and economists are expecting an annual rate of 6.12 million units versus 5.77 million the month before.

The Federal Reserve Bank of Richmond's June indexes on the region's manufacturing and service sectors are also expected at 10 a.m. (1400 GMT) For details, see

S&P 500 futures were off 4.8 points and below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures slipped 26 points, and Nasdaq 100 futures lost 4 points.

Oil services companies waited to see if they win a legal fight to overturn a six-month ban on deepwater drilling in the Gulf of Mexico. A U.S. judge will decide by Wednesday whether to temporarily lift the ban.

Large banks looked likely to face new limits on debit-card transaction fees as a key U.S. senator said he reached a deal to include the provision in an overhaul of financial regulations. But the new deal scales back the limits on fees card networks like Visa Inc and MasterCard would have faced.

Drugmaker Pfizer Inc is pulling a decade-old leukemia medicine off the U.S. market after a study found a higher death rate and no benefit for patients.

President Barack Obama is expected to warn health insurance executives at a White House meeting against imposing big rate increases ahead of tighter rules under the new healthcare law, The New York Times reported.

Stocks ended lower Monday, once again succumbing to late-day selling in light trading.

(Editing by Padraic Cassidy)