Court documents show the companies are renegotiating the terms of the sale, but neither party will disclose what sparked the renegotiations or what the new terms will entail, according to the Associated Press.

The $57 million deal would see Ambre Energy take 100% control of the Decker mine. It was set to be completed on April 1.

The deadline was postponed to May 10, a move that will force Ambre Energy to pay Cloud Peak a further $7 million for the delay.

The dispute over the acquisition is currently being heard in the US District Court of Montana and will now be extended until July 12.

"The underlying transaction is complicated and the parties are renegotiating the terms of the proposed sale due to unforeseen complications," attorneys for the companies wrote, according to the AP.

Cloud Peak declined to provide the news service with any information about the deal but referred to a statement in its May 1 quarterly report that said the deal would close in the first half of 2013.

Ambre Energy told the AP the company had “no doubt” the deal would come together in coming months.

The Decker mine has produced more than 9 million tons per annum previously but is currently producing at 3Mtpa.

Ambre plans to ramp production back up to full capacity and export the coal through a pair of proposed west coast export terminals to growing Asian markets.

However, US energy analyst Sightline Institute said in February that Ambre’s foray into US exports looks unlikely to be successful amid a number of money woes for the company.

According to the Sightline Institute report, Ambre faces mounting financial challenges, including money-losing coal mines, large write-offs for failed overseas ventures, major liabilities for mine cleanup and pensions, troubled assets, high borrowing costs and a need for $1 billion in new capital to make its coal projects financially viable.

The proposed pair of coal export terminal sites in Washington and Oregon, were expected to begin first shipments in 2014.