Watch for a new name on the dollar bill. The U.S. treasurer, Angela Buchanan (better known to friends as "Bay"), is leaving the Treasury Department July 1 to return to California to be closer to her family, which includes a new baby.

John Kelly, assistant secretary for public liaison and consumer affairs, is also leaving to join Worldwide Information Resources Ltd., a consulting group here run by Richard Whalen.

Treasury Secretary Donald T. Regan brought Kelly with him to Washington from Wall Street. Kelly had been public affairs chief for Regan when the secretary was heading the bulls at Merrill Lynch and Co. Inc..

But by the time Kelly reached Washington in the spring of 1981, Ann McLaughlin--wife of TV panel host John McLaughlin--had made herself indispensable to Regan. With McLaughlin running the traditional public information office as assistant secretary for public affairs, Kelly had a new job created for him.

He has kept out of the way of the press during his two years in Washington, but has handled Treasury relations with business and trade and financial groups, including old acquaintances from Wall Street. Kelly leaves officially on July 1, but he took off on vacation last week. No replacement has been named yet. TREASURY'S LOSS, IMF'S GAIN . . . Treasury is also losing a staff member to one of the key international agencies in town: the International Monetary Fund. Thomas B. Leddy, deputy assistant secretary for international monetary affairs, is off to the IMF as assistant treasurer.

The IMF spotlight recently has focused on the role the fund is playing in the Third World's debt crisis. At Treasury, though, Leddy was responsible primarily for industrialized nations, with Thomas C. Dawson as the deputy assistant secretary in charge of worrying about developing nations. Meanwhile, Charles Dallara has come back from the IMF to take Leddy's place at Treasury. Before his stint at IMF as one of the U.S. representatives, Dallara was special assistant to Marc Leland, who as the assistant secretary for international affairs will now be Dallara's boss again. EGGER UPDATE . . . Internal Revenue Service Commissioner Roscoe L. Egger Jr. has had open heart surgery and will be out of the office through the summer. Deputy James I. Owens is in charge, and will no doubt be keeping an eye on what Congress is planning for our tax rates.