* S&P 500 index futures below fair value

* Price of oil rises above $115 per barrel

* Existing home sales data for July on tap

By Kristina Cooke

NEW YORK, Aug 25 (Reuters) - U.S. stock index futures slipped on Monday, with Wall Street set to give back some of Friday's sharp gains, as the price of oil rebounded and credit concerns nagged.

Worry about the impact of higher fuel costs on consumer spending and profits flared up again as the price of oil rose. Shares of retailers such as Wal-Mart (WMT.N: Quote, Profile, Research) and airlines, including Continental Airlines (CAL.N: Quote, Profile, Research), fell before the bell.

Lehman Brothers LEH.N shares fell 4.2 percent before the open after a top South Korean regulator voiced concern about state-run Korea Development Bank's interest in buying a global bank. For details, see [ID:nSEO367339].

Shares of Lehman Brothers, which has been hard hit by the credit crisis, had soared on Friday after KDB said it was open to the acquisition of an overseas financial institution, naming Lehman as one option.

Any news on when and whether the U.S. Treasury will take steps to rescue mortgage finance giants Fannie Mae (FNM.N: Quote, Profile, Research) and Freddie Mac (FRE.N: Quote, Profile, Research) could determine market direction, analysts said.