The court deadline already had been extended once, from an original date of April 1.
A Cloud Peak spokesman declined to say what parts of the deal were up for renegotiation. Spokesman Rick Curtsinger referred to a statement in Cloud Peak's May 1 quarterly report that said the deal would close in the first half of 2013.
Ambre spokeswoman Liz Fuller said the Australian company has "no doubt" the deal will come together in coming months, She added that the company has proven it can raise funding after coming up with money for other mine acquisitions and its two port projects.
In 2011, Ambre bought 50 percent stakes in Decker and a second mine, Wyoming's Black Butte, that it owns with Anadarko Petroleum. Ambre last year announced a deal to supply up to 5.5 million tons of coal per year to a pair of utilities in South Korea.
Decker was once among the largest coal strip mines in the U.S., and produced more than 10 million tons annually as recently as 2002.
But domestic demand for coal has been on the decline due to competition from cheap natural gas and the costs of making older power plants compliant with environmental regulations.
In December, Decker laid off 59 employees — more than one-third of its workforce. The mine is on track to extract only 1.5 million tons of coal this year, based on first quarter production figures reported to the U.S. Department of Labor.
Australian media last year reported that Ambre was at risk of financial collapse after a proposed coal mine and coal-to-liquid fuels plant it was pursuing in the country was rejected by government officials.
The Sightline Institute, a Seattle-based environmental think tank, in February released a report that revealed Ambre racked up losses of more than $124 million from 2005 through 2012.