But, anything Mr. Kushner does at 666 Fifth — renovate and sign new leases for the office space, create condominiums or some combination of the two — could cost hundreds of millions of dollars, at a time when most analysts say the building may not be worth as much as its mortgage.

“It’s tough. It’s an iconic asset in a prime location, but it probably needs a substantial investment to compete with newer, shinier office product at Hudson Yards, downtown or near Grand Central,” John P. Kim, a real estate analyst at BMO Capital Markets, said of 666 Fifth.

Stephen B. Siegel, chairman of global brokerage at CBRE Group, a real estate company, said, “It’s an eminently leasable office building, if updated to modern standards, from the lobby to the elevators.”

But upgrades cost money, potentially $200 million, Mr. Siegel said.

As for luxury condominiums, Jonathan J. Miller, a residential real estate appraiser, said that the challenge is that “the high end of the residential market has been overbuilt since 2014.”

Very few Midtown office buildings trade hands so it is difficult to make a comparison between 666 Fifth and other deals. But real estate executives say that the building, which has a 30 percent vacancy rate, is worth anywhere from $800 a square foot to $1,000 a square foot, or as much as $1.4 billion — the amount of the mortgage.

The clock is ticking on that loan, which comes due in February. Revenue from the building only covers about half of the annual payments, according to a financial report by Trepp, which tracks real estate debt.

If he refinanced, Mr. Kushner could probably get a new mortgage at a lower interest rate than the current 6.3 percent. But experts say that Mr. Kushner would only be able to finance about 80 percent of the building’s value, or about $1.12 billion.