Donald Trump cannot find a private company to guarantee the $464m (£365m) he has been ordered to pay in a New York civil fraud case.
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The former president must either pay the full amount in cash or secure a bond to avoid having assets seized.
If he is unable to do so by March 25, he faces the prospect of some of his real estate assets being seized by the state's attorney general.
Mr Trump said that securing a bond of that size was "practically impossible".
For a fee, a bonding company would guarantee the full amount to the New York court. They would then have to pay it if Mr. Trump loses his appeal and cannot do so himself.
In his statement, Mr Trump said that the bond he was asked to pay would be "impossible for any company, including one as successful as mine".
"The bonding companies have never heard of such a bond, of this size, before," he said.
Mr Trump's team spent "countless hours negotiating with one of the largest insurance companies in the world", his lawyers wrote in a court filing.
But they concluded that "very few bonding companies will consider a bond of anything approaching that magnitude". To secure a bond, an individual has to demonstrate to the company providing the guarantee that they have enough liquidity - usually in the form of cash plus things like stocks or securities which can be sold quickly - to cover the amount.
Mr. Trump's two eldest sons must also pay millions of dollars in the case.
Along with ordering Mr. Trump to pay the penalty, New York Judge Arthur Engoron banned him from running any businesses in the state for three years after he found the former president falsely inflated assets to secure better loan deals.