Donald Trump was thrown a huge lifeline Monday when an appeals court slashed what could have been a crippling demand for a nearly half-billion-dollar bond payment and also gave him 10 more days to pay.

The Republican presidential candidate got the unexpectedly positive news about his New York civil fraud case while he was sitting in court for another case -- a hearing in his upcoming criminal trial over paying hush money to a porn star.

Trump had been facing a Monday deadline to pay the huge original bond pending an appeal against a judge's decision that he is liable for fraudulently conspiring to inflate his net worth.

Trump had made clear he was unable to find the US$454 million and he risked seeing New York state confiscate parts of his property empire in response. Now with Monday's decision, he will have 10 more days to pay just US$175 million.

Writing on his Truth Social app, Trump said he would "abide by the decision of the Appellate Division and post either a bond, equivalent securities, or cash."

The 77-year-old Republican began his day of twin legal crises in a Manhattan courtroom at a hearing to set a new date for his trial over payments he made on the eve of the 2016 presidential election to make sure porn star Stormy Daniels did not publicize a sexual encounter.

"This is a witch hunt," Trump told reporters before walking into court.

The hush money trial had been scheduled to begin on Monday but was delayed at the last minute because tens of thousands of pages of potential evidence were belatedly produced by prosecutors.

The Manhattan district attorney has told Judge Juan Merchan he would accept a delay of up to 30 days for the start of the trial while lawyers for Trump have asked for at least three months.

On Truth Social, Trump denounced both cases as a politically motivated attack ahead of the November 5 presidential election when he will likely again face incumbent Democrat Joe Biden.

"These are Rigged cases, all coordinated by the White House and DOJ for purposes of Election Interference," Trump wrote. "No crime. Our Country is CORRUPT!"

Trump regularly rails against a judicial system he claims is "fixed" against him.

He has said that New York Attorney General Letitia James -- who is Black -- is "racist," and accused Judge Arthur Engeron in the civil fraud case of being "crooked."

Trump got some positive financial news last week when it was announced that Truth Social would finally go public through a merger, a transaction that could net him billions of dollars.

He cannot tap into the funds for six months, but it potentially could help him secure a bond.

However, in case of non-payment, James could still order a freeze on his bank accounts, or move to seize some of his New York properties.

Trump's lawyers have pursued every avenue to delay his many trials -- if possible until after the presidential vote.

In the trial involving Stormy Daniels, whose real name is Stephanie Clifford, Trump is accused of illegally using campaign funds to secure her silence about an alleged sexual encounter back in 2006.

If she'd spoken out, the fallout could have been ruinous for Trump's presidential hopes, coming right on the heels of another scandal over the married businessman's boasting, caught on tape, that as a celebrity he was freely able to "grab" women by their genitals.

Trump is accused of making the hush money payments in an illegal manner involving falsification of business records.

The former president potentially could face a prison sentence of up to four years if convicted. (AFP)