Bank of America's four branches in Paraguay have been sold to a group of Paraguayan investors . . . Hilton Hotels Corp. said it approved additional investments of $100 million at its Las Vegas properties . . . A group led by Metromedia Co. said it raised its stake in Orion Pictures Corp. to the equivalent of 24% of the common shares outstanding . . . Charles Schwab & Co. said it reduced its work force by 5%, affecting 160 employees throughout the United States . . . The Federal Reserve said the average yield for one-year Treasury bills, the most popular index for making changes in adjustable-rate home mortgages, fell to 6.77% last week after averaging 6.90% the week before . . . Allegis Corp. said the previously announced sale of its Westin Hotels & Resorts to Caesar Park Hotel Investment Inc. and Robert M. Bass Group has been completed . . . Dart Group Corp. undertook a hostile $865-million takeover bid for Stop & Shop Cos., a grocery-discount chain that rejected a slightly cheaper offer last week from the retailing concern.