A measure of future house sales picked up last month in Greater Des Moines, with 793 contracts to purchase a house being written, compared with 754 in July and 641 in August 2010, the Des Moines Area Association of Realtors said today. Real estate agents closed the deal on 723 contracts in August, down from July, but an increase from the 546 closings reported in August 2010. Houses sold in August were on the market an average of 112 days and there were 5,534 active listings. The median sale price was $149,500. Buyers paid with cash in 157 of the transactions in August, the association said.