Robinson Terminal Warehouse, which is owned by The Washington Post Co., plans to build a port with a 2,000-foot pier and a warehouse on a historic bluff of the Potomac River on Cherry Hill Peninsula in Prince William County.

Riggs Bank's subordinated debt rating was downgraded by Moody's, which cited Riggs's exposure to weakening commercial real estate markets.

Mack Trucks, which reported a $63 million loss in the second quarter yesterday, gained a second suitor -- Navistar International Corp. The interest from Navistar came two weeks after Renault Vehicules Industriels, which owns 44.6 percent of Mack stock, announced it wanted to acquire the remainder of Mack stock it doesn't own for $98 million. Navistar said it has requested additional information from Mack to determine whether to make an offer and at what price.

Potomac Mills, the world's second-largest discount mall, will be 60 percent bigger by 1992, mall officials said. Plans for the Prince William County mall, owned by Western Development Corp., call for five large anchor stores, including J.C. Penney, and about 48 small shops. The expansion is expected to bring 1,500 new jobs to Prince William County.

Lawrence A. Hough was named president and chief executive of the Student Loan Marketing Association, replacing Edward A. Fox, who resigned to take a job in academia. Hough had been executive vice president for marketing, servicing and systems since 1984 and has been with Sallie Mae since 1973.