When billionaire financier Warren Buffett announced he was assuming the chairmanship of Salomon Brothers on an interim basis last week, he stepped into a morass that threatened to grow worse for the 81-year-old Wall Street firm before it got better. In fact, Buffett's salvage job began even before he was able to warm his new seat. The Treasury Department, in an attempt to restore confidence in the market, barred Salomon from bidding at further auctions. In a series of telephone calls with vacationing Treasury Secretary Nicholas Brady, Buffett successfully lobbied for leniency. Salomon was permitted to trade, but for its own account only, not on behalf of clients. The decision was more than symbolic, since Salomon, one of only 40 firms designated as primary dealers in T-bonds and T-bills, directly and indirectly counts on government securities for about 25% of its business. The firm participated in last week's auction under the watchful eye of Treasury officials. Buffett, who owns 16% of Salomon's preferred stock and a legendary reputation for his investing, if not his investment-banking, savvy, assumed Solly's chairmanship after the board forced chairman John Gutfreund and two other top executives to step down. Buffett immediately brought in Deryck C. Maughan, 43, who until recently ran Salomon's Asian operations from Tokyo, and jettisoned two bond traders. Executives admitted that the firm had violated the rules that prohibit any one bidder from buying more than 35% of a single issue at a Treasury auction, and that they had skirted regulations barring a firm from submitting bids in its customers' names without their authorization in order to conceal such illegal efforts to influence the market.