The owners of New York’s new professional soccer team and the Bloomberg administration are nearing a tentative agreement to build the city’s first soccer-only venue: a $350 million, 28,000-seat stadium near Yankee Stadium in the Bronx.

But it is far from a done deal.

Mayor-elect Bill de Blasio, who was not briefed on the deal until Wednesday, did not immediately embrace the proposal, in part, because the Bloomberg plan entails tax breaks, the sale or lease of public land and public financing.

Lis Smith, a de Blasio spokeswoman, said, “We have real concerns about investing scarce public resources and forgoing revenue to support the creation of an arena for a team co-owned by one of the world’s wealthiest individuals, and will review any plan with that in mind.”

The team, now starting to take shape and known as the New York City Football Club, is a partnership between the Manchester City Football Club of England, which is owned by the billionaire Sheikh Mansour Bin Zayed Al Nahyan, and the Yankees.