But when their closing date arrived last September, several banks told them that to get a mortgage, they would have to increase their 10 percent down payment by another 15 to 25 percentage points. With no way to come up with that much money, the Phams notified the developer, Toll Brothers, that they could not get financing for the apartment. Toll Brothers declared them in default and kept their deposit.

“It would take us another 15 years to save that money again,” Ms. Pham said.

The Phams, who have two children, a 4-month-old and a 2 ½-year-old, live in a two-bedroom in Hoboken that is smaller than the one they had hoped to move into in Maxwell Place. But they borrowed on their equity there to help put together the deposit on the new apartment. The rest of the deposit came from Mr. Pham’s work as a real estate agent, income that has all but dried up in the current market.

“If we tried to sell our apartment, we wouldn’t make enough to cover the cost of selling it, so we’re really stuck,” said Ms. Pham, who works as a benefits manager at a professional services firm. Ms. Pham said that the developer “made no attempt to work with us; they wouldn’t even return my phone calls.” She added that the sales manager had declined their offer to help find another buyer for the apartment and had told Ms. Pham that “not getting our deposit back was just business.”

A spokeswoman for Toll Brothers declined to comment because the Phams have filed a lawsuit to try to get their money back.

Brokers and developers say that every new development probably has a handful of buyers who are facing this difficulty. (The Web site Streeteasy.com lists 460 new developments with active sales listings in Manhattan and 438 in Brooklyn.)