One of Neiman Marcus' debt investors claims the struggling luxury department store may be insolvent and in default on its bonds after shuffling around some assets earlier this month.

Marble Ridge Capital condemned Neiman Marcus for transferring its crown jewel e-commerce business, MyTheresa, away from the grasp of its bondholders and into its parent company, according to a Sept. 18 letter Marble Ridge founder Daniel Kamensky sent to the retailer's board of directors.

"These recent actions threaten the viability of a storied franchise that includes marquee brands such as Neiman Marcus and Bergdorf Goodman," Kamensky wrote.

The luxury retailer has been grappling with roughly $4.7 billion in long-term debt as it navigates the rapidly changing retail landscape. The business, owned by Ares Management and the Canada Pension Plan Investment Board, has explored a number of strategic options to cope with that debt, including a sale.

Neiman Marcus doesn't deny the change. It disclosed the restructuring on Sept. 18. In doing so, it follows a move made by a number of its peers, including J. Crew, that have stripped a company's most valuable assets away from its debt-holders, thereby provoking a legal battle.

Neiman Marcus maintains the company had the right to transfer the business because MyTheresa wasn't being used as collateral for its bonds.

"As publicly disclosed, MyTheresa was already an unrestricted, non-guarantor subsidiary not part of our lenders' collateral and it will remain outside of the collateral. This reorganization was expressly permitted by the company's credit documents," a Neiman Marcus spokesperson told CNBC in an emailed statement.

Marble Ridge, however, believes Neiman Marcus improperly categorized its MyTheresa business as unrestricted, meaning it can do whatever it wants with it.

The fund also maintains that the company could have been insolvent when it transferred the MyTheresa, potentially causing it to have defaulted on its debt.

"Rather than allowing the theft of assets by CPPIB and Ares, we believe a more responsible board, given its fiduciary obligations, would have engaged in a strategic review to maximize value for the benefit of all of the company's stakeholders."

Marble Ridge is urging Neiman Marcus to sell certain assets to pay down its debt, in lieu of its transfer of MyTheresa.

Neiman Marcus earlier this month reported $4.9 billion in sales for its fiscal year ending July 2018 and net earnings of $251 million.

Ares and CPPIB did not immediately return requests for comment.

Correction - This story has been corrected to more accurately reflect Marble Ridge's claims against Neiman Marcus. It also previously inaccurately said that it is urging Neiman Marcus to sell Bergdorf Goodman.