Manhattan is getting an unwanted taste of South Florida. Developers of new condominiums are finding that apartments they thought had sold are unexpectedly coming back into their hands as buyers — not just layoff victims, but some who are wealthy and employed — default on contracts. In some cases, these supposed buyers are having to walk away from five- and six-figure deposits. This phenomenon, common in Florida and other real estate markets decimated by the housing slump, was virtually unheard of in New York City until recently.

“Inventory that’s considered 100% sold out is now trickling back on the market,” a mortgage broker at Commodore Mortgage Group, Andrea Costa, said. “Before, you had people lining up for these apartments. This is definitely new, especially for New York.”

At 1 Hanson Place in Fort Greene, a family recently gave up its $70,000 down payment after the father became unemployed, according to Brian Huang of City Connections Realty, the firm that represented the buyer. A mortgage broker at the Thomas Funding Group, Thomas Wiggin, said one of his clients gave up a $75,000 down payment on an apartment in Brooklyn after finding that the financing he’d counted on is no longer available in the face of tighter lending restrictions. Many buyers of new-construction condominiums, where a year or more often goes by between the signing of contracts and closing, are facing the same problem, mortgage brokers say.

The resulting spike in inventory, combined with expected layoffs on Wall Street, could have far-reaching effects on the city’s real estate market and the economy, including foreclosures and heavily discounted prices.

The senior vice president at Preferred Empire Mortgage Company, Jeffrey Appel, estimated that the number of buyers struggling or failing to close on signed contracts leapt to 10% from virtually nil in the past year. “From zero to 10% is a big difference,” he said. “It’s a growing problem.”

Buildings where buyers are scrambling to close include some of the city’s most luxurious new condominiums, including 101 Warren St. in TriBeCa, the Zinc Building on Greenwich Street, and DISTRICT on Ann Street, sources said.

“It’s happening all over the city,” Mr. Wiggin, said. “No building is immune.”

A year ago, banks routinely allowed buyers to finance up to 90% of their home purchases, but now require a down payment of 20% or more, Mr. Wiggin said. Since mortgage contingencies usually last only a month, buyers who signed contracts more than a year ago are discovering at closing that they must come up with large sums of cash to make up the difference.

“If you got pre-approved a year or two ago, there’s a 95% chance that your financing is no longer available,” the chief executive of Manhattan Mortgage Co., Melissa Cohn, said. Because of the high cost of New York City housing — in Manhattan, the price of the average apartment now exceeds $1 million — an additional 10% of the purchase price is difficult to acquire on short notice, even for wealthy buyers. “What’s amazing is that if you look at these people and their income, they’re able to afford these apartments,” Ms. Costa said. “The lending isn’t based on qualifications — it’s a set of rules because we got into this credit mess.”

If buyers can’t come up with the cash, they could lose an even larger amount on their deposit. “You could be walking away from $300,000 — that’s not chump change,” Ms. Costa said.

Buyers are increasingly using gifts from family members to help make up the difference, or looking for more creative ways of financing their purchase.

The president of RES Real Estate Services, Dogan Baruh, said he recently found a group of private investors to help a client come up with an additional 25% on a $900,000 TriBeCa condo after his original financing fell through. Before the deal worked out, “the client was almost ready to walk away from the purchase and lose his deposit,” Mr. Baruh said. “It’s almost better to lose $100,000 than to have to come up with an additional $200,000.”

Several buyers at 101 Warren St. have negotiated with the developer, requesting more time to close, Benjamin McGrath, the chief financial officer of the building’s developer, Edward J. Minskoff Equities, said. “We’ve had a couple who have had difficulty making it to the closing table, but they’ve all come through in the end,” he said. The building, which has only one penthouse left for sale, has not yet closed all of its units.

The developer of DISTRICT, Joseph Klaynberg, said several buyers at the building entered into contracts with a 10% deposit, only to discover that they couldn’t get financing for the remaining 90%. Since they pulled out of the deal very early on, the developer refunded their money, he said. The building is now 88% sold, he said, and closings are expected to begin soon.

Developers are not legally obligated to return deposits after a contract has been signed, according to real estate attorney Keith Schuman, who said he has several clients who likely will sue to recoup their deposits.

Buyers in danger of defaulting may find other parties to take over their contracts, but that’s a difficult proposition in the current market, a broker at City Connections Realty, Brian Huang, said. The family who lost its down payment at 1 Hanson Place tried unsuccessfully to find another buyer, he said.

Meanwhile, those that haven’t yet lost money are scrambling to avoid doing so.

“Brokers in buildings all across the city have people that are not closing,” Mr. Wiggin said. “I get calls from people who say, ‘I can’t get financing. Can you help me?'”