Goldman Sachs Group Inc. was fined $800,000 by the Financial Industry Regulatory Authority for failing to have procedures in place to ensure clients in its dark pool got the best price while trading stocks.

More than 395,000 transactions were executed in the New York-based bank’s Sigma X pool at an inferior price to the national best bid and offer from July 29 to Aug. 9, 2011, Finra said today in a statement. Stock venues in the U.S. are obliged to price trades at the NBBO or better.

Goldman Sachs discovered the pricing errors and sent checks to customers to compensate for the mistakes, people with direct knowledge of the matter said in March. Finra said today that the firm returned $1.67 million to affected customers.

Goldman Sachs was unaware of the errors at the time, according to the Finra statement. The Standard & Poor’s 500 Index tumbled 13 percent through the first eight days of August 2011, while the Chicago Board Options Exchange Volatility Index hit a multiyear high of 48 on Aug. 8.

Goldman Sachs neither admitted nor denied Finra’s charges. Tiffany Galvin, a spokeswoman for the bank, declined to comment.