NEW YORK, SEPT. 28 -- Spurning an unsolicited offer from Revlon Inc., Salomon Inc.'s board of directors tonight unanimously approved a proposal by investor Warren Buffett to pay $700 million for a 12 percent stake in the Wall Street firm.

Revlon is unlikely to let up pressure on Salomon, however. Revlon said earlier today that if its offer to buy the same 12 percent block offered to Buffett was rejected, it might acquire Salomon common stock on the open market.

Under pressure from Revlon's overture, the Salomon directors met for three hours tonight to consider proposed agreements with Buffett and Mineral and Resources Corp. Ltd., which Sunday agreed to sell its 14 percent stake in Salomon back to the firm for $809 million. Sources said that Revlon representatives telephoned several times during the board meeting to urge the directors to pursue negotiations with Revlon.

The Salomon board unanimously rejected Revlon's proposal. Sources said that individual directors voiced concern during the meeting that an alliance with Revlon and its chairman, Ronald Perelman, who is known for his aggressive takeover strategy, might alienate the Wall Street firm's corporate client list.

Revlon proposed earlier today to match closely the offer made by Buffett's Berkshire Hathaway Inc. to purchase preferred stock in Salomon. But it said it would pay $4 more per share if it converts the preferred shares into common stock in three years.

Revlon also today asked the federal government for clearance to buy more than $15 million worth of Salomon common stock, in the event its proposal to buy preferred stock was rejected. Wall Street sources said Revlon purchased about $7.5 million in Salomon stock today.

Salomon chairman John H. Gutfreund met last week with Revlon chairman Perelman, but decided he did not want to pursue an association with Revlon, sources said.

Wall Street sources said Revlon regards the terms of Buffett's investment in Salomon as exceptionally favorable to Buffett.

The Omaha investor, who holds large stakes in Capital Cities/ABC Inc., The Washington Post Co. and Geico Corp., has agreed to pay $700 million for new preferred shares to be issued by Salomon that will yield a 9 percent annual dividend. The preferred stock can be converted into common stock after three years at $38 per share, giving Buffett a 12 percent stake in Salomon and making him the firm's largest shareholder. Revlon today offered the same terms but said it was willing to pay $42 per share if it converted its preferred stock to Salomon common.

Buffett's $700 million will be used to help finance the repurchase by Salomon of $809 million of stock held by Minerals and Resources (Minorco), which is controlled by the South African conglomerate Anglo-American Corp.

Wall Street sources described the proposal by Revlon today as a disenchanted response to a series of failed talks last week involving Revlon, Minorco and Salomon.

According to Wall Street sources, Lazard Freres & Co. partner Felix Rohatyn, who was representing Minorco, alerted Gutfreund about talks between Minorco and Revlon, which had expressed interest in buying Minorco's stake. Lazard arranged a meeting between Revlon Chairman Perelman and Gutfreund. Gutfreund decided that he did not want to pursue an association with Revlon.

Revlon's approach prompted the Salomon chairman to turn to Buffett, the sources said. As previously reported, Gutfreund and Buffett met in New York last Tuesday and initiated talks that culminated in their investment agreement.

Last Saturday, Gutfreund took the final step and asked Minorco to sell its shares back to Salomon. He offered to pay Minorco the same price that Revlon had earlier offered for the Salomon shares.

Wall Street sources said Minorco accepted Gutfreund's offer even though some of those involved in the talks felt Revlon might increase its offer if pushed.

Before Revlon's latest offer was made public today, Gutfreund said he was extremely pleased about his deal with Buffett. "Warren's a terrific guy," Gutfreund said. "The major advantage to me is that he's a very bright, successful man whom I know can give me a disinterested opinion."

Salomon is in the midst of a major reevaluation of its businesses, which Gutfreund said he expects to complete toward the end of the year. Like some other large Wall Street firms, Salomon has experienced extraordinary growth during the last six years.