Pending home sales retreated in August after hitting a two-year high in the previous month, a trade group reported Thursday. . The pending-home-sales index fell to 99.2 from a upwardly revised 101.9 in July, the National Association of Realtors said The NAR initially said the July index was at 101.7. “The performance in month-to-month contract signings has been uneven with ongoing shortages of lower prices inventory in much of the country,” said Lawrence Yun, chief economist of the NAR. Compared to the same period in 2011, pending home sales were up 10.7%, marking the 15th straight month of year-on-year gains. A sale is listed as pending when the contract has been signed but the transaction has not closed, and an index of 100 is equal to the average level of contract activity during 2001.