The Metropolitan Transportation Authority could lose tens of millions of dollars if its tentative deal with Tishman Speyer to develop the West Side rail yards collapses, according to a number of sources.

Representatives of the MTA and Tishman Speyer are scheduled to meet today to try to salvage the deal. A failure to resolve their differences could lead the MTA to reopen the bidding process, or possibly approach one of the runner-up bidders. Both scenarios, observers say, leave the MTA with reduced bargaining leverage.

“The MTA will not be in the greatest position if Tishman backs out,” a source with knowledge of the negotiations, who asked not to be identified, said. “Obviously the bids would go down.”

With a winning bid valued at just more than $1 billion, Tishman Speyer edged out the joint venture of the Durst Organization and Vornado Realty Trust. When the deal was originally announced in March, executives from the MTA said Tishman Speyer’s bid was $112 million more than the Vornado-Durst bid. The other bidders involved in the process were Extell Development Co., the Related Cos., and Brookfield Properties.

The MTA announced Thursday that negotiations with the developer had reached an impasse. Tishman Speyer was seeking guarantees on the extension of the no. 7 subway line and assurances that the city would approve the rezoning of the western half of the rail yards without major changes. The eastern section of the rail yards has already been rezoned, but the western half, where the Bloomberg administration had wanted to build a football stadium, still needs to approved through the city’s land-use review process.

Over the weekend, Mayor Bloomberg said he was optimistic that a deal could be reached. Mr. Bloomberg met with one of the founding partners of Tishman Speyer, Jerry Speyer, in London to address concerns about the 1.5-mile extension of the no. 7 subway line, which would give the Hudson Yards improved transit access.

“Tishman Speyer’s not stupid,” Mr. Bloomberg said in a radio interview. “I think we satisfied them. The city has done everything it can. We don’t run the MTA. The MTA’s a state agency.”

The president of the Real Estate Board of New York, Steven Spinola, said it was too early to tell how a new bidding process would play out, but said the underlying issues would need to be resolved. He said other bidders would likely share Tishman Speyer’s concerns about rezoning and the subway line extension.

Assemblyman Richard Brodsky, chairman of the Committee on Corporations, Commissions and Authorities, introduced legislation on Friday that would establish a new state authority to oversee how the rail yards are developed.

Mr. Brodsky’s legislation would effectively wrest oversight of the rail yards away from the MTA by allowing the new authority to borrow money to purchase the 24-acre site. The MTA would still receive funds for its capital plan. Mr. Brodsky said the new authority would then sell the land off parcel by parcel, a model borrowed from the development of Battery Park City.

“This deal ought not to go forward and we ought to turn toward a model that works,” Mr. Brodsky said yesterday. “We should not be selling a public asset at this price at this time.”

In a statement, Jerry and Robert Speyer said: “We still hope to be able to complete this deal and reach an agreement that satisfies the needs of everyone.”