Mayor Bloomberg, who has had a long string of successes in dodging conflicts of interest with his multibillion-dollar business holdings, is suddenly being confronted with a case that may upend his luck.

Merrill Lynch, the company that wants to unload its 20% stake in Bloomberg LP for as much as $6 billion, also figures in the development debate at ground zero, where the mayor is a major player. If Merrill gets an offer for its interest in the mayor’s company, it must offer him a right of first refusal.

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If Mr. Bloomberg exercises that right, he would be in the position of negotiating the price and terms of a multibillion-dollar business deal with Merrill Lynch, a major New York City employer whose real estate moves could affect the future of a key Lower Manhattan development project, the rebuilding of ground zero.

If Mr. Bloomberg chooses not to exercise the right of first refusal, Merrill Lynch could be in the position of deciding who will be the new 20% minority stakeholder in a business that constitutes most of the mayor’s net worth. And if the mayor sees the Merrill Lynch deal as an opportunity to take Bloomberg LP public or to somehow otherwise liquidate his 68% stake in the company, the transaction could underscore for the public the vastness of Mr. Bloomberg’s personal wealth. Forbes estimated it in 2006 at $5.3 billion, but a New York Sun article that year pegged it at closer to $20 billion, a figure that is borne out by Merrill’s estimate of the value of its 20% stake.

“There has always been a question about the length-of-the-arm distance between the mayor and his company,” a professor of political science at Baruch College, Douglas Muzzio, told the Sun yesterday. “The sale of Merrill Lynch’s stake could revive this issue in the minds of some people.”

Merrill Lynch’s chief executive, John Thain, last month valued the company’s ownership stake at as much as $6 billion, placing Mr. Bloomberg’s stake at $21.6 billion — not accounting for a premium for the mayor’s majority control. In the midst of an economic downturn, public knowledge of this sum could make the mayor seem elitist and out of touch with voters, should he decide to attempt a run for governor or a third term as mayor.

In a 2002 decision, the city’s Conflicts of Interest Board agreed that the mayor would have to recuse himself for any dealings the city had with Merrill Lynch, but that the mayor could be involved in important changes to the company.

Mr. Bloomberg “advises that he will have no involvement in the day-to-day operations of Bloomberg L.P., but wishes to maintain the type of involvement that he believes is consistent with being the majority shareholder,” the agreement said. “In particular, Mr. Bloomberg proposes to remain involved in those business matters that might significantly affect the value of his ownership interest.” This includes the sale of the company or of a significant interest, the sale or purchase of a major asset by the company, or a major financial commitment, such as the acquisition of debt, the agreement said.

The Conflicts of Interests Board advised the mayor “to be sensitive” as to whether a company that Bloomberg LP is negotiating with is engaged in any business dealings with the city, should Mr. Bloomberg become involved. If there is the possibility of a conflict, the mayor should “seek the board’s advice on a case-by-case basis.”

The Conflicts of Interest Board declined to comment on whether the mayor would have to appear before the board should he wish to buy back a piece of Bloomberg LP or involve himself in the sale of Merrill Lynch’s stake.

“Your premise is fundamentally incorrect: the Mayor doesn’t discuss City business with Merrill Lynch because he has a personal business relationship with them,” a spokesman for the mayor, Stuart Loeser, said.

Merrill Lynch is one of the largest employers in Lower Manhattan and is considering moving its headquarters to ground zero, where it will likely look for incentives and tax breaks similar to the billions of dollars Goldman Sachs received when it built its headquarters at Battery Park City.

Because Merrill Lynch is a public company, any sale of the portion of Bloomberg LP, which is private, would be publicly recorded, rendering it nearly impossible to keep a lid on much of the mayor’s personal wealth. Regardless of whether Mr. Bloomberg buys Merrill Lynch’s stake, it is unlikely that any sale would be completed without the mayor’s approval.

According to the right of first refusal, if Merrill Lynch finds a party that is interested in acquiring its stake in Bloomberg LP, the company — and therefore the mayor — has the option of matching the sale price and buying the 20% itself. It is unlikely that Merrill Lynch, which provided seed money to Bloomberg LP in 1981 and was the company’s first customer, would sell to any party that is not first approved by Mr. Bloomberg.

“Whenever the mayor decides to sell the company, given the fact that Merrill Lynch has been so involved, it wouldn’t want to anger the mayor and risk the chance of not being the bank on the sale and earning the potentially $1 billion in fees,” a person familiar with the deal said.

Because Merrill Lynch’s stake is not a controlling share of the company, likely buyers, if not Mr. Bloomberg himself, would include strategic players in the news industry interested in having an inside look at the way the private company operates and taking advantage of content sharing. A buyer could also be a financial services company interested in the dividend payments that the company makes, experts said.