Legal action is being taken to shift insolvency proceedings for budget airline Niki to Austria from Germany, in a move that could endanger the sale of the Air Berlin subsidiary to International Airlines Group.

Fairplane, a group representing airline passengers, is reported to be seeking to recover more than €1 million it claims Niki owes to passengers, filed separate legal cases on Tuesday to block insolvency proceedings in Berlin and to open them in Austria instead.

Although the sum sought is relatively small, a spokesman for Air Berlin liquidator Lucas Floether said the complaint could derail the sale of Niki to IAG’s low-cost Vueling operation that was agreed last week.

“If the complaint before the Charlottenburg Court (in Berlin) succeeds, the sale of Niki to IAG would be greatly endangered,” the spokesman told Reuters. A ruling by the court on the case was due this week.

British Airways’ owner IAG announced plans late on Friday to acquire Niki for €20 million and provide up to €16.5 million in additional liquidity to the carrier.

The sale to IAG, which had been in exclusive talks to acquire the airline, appeared to mark the final chapter in the demise of Air Berlin, the former second largest German air carrier that earlier owned Niki.

But Fairplane argues that Niki, which is registered in as a company in Austria, had been profitable but had lost access to bridge financing when insolvency proceedings were opened in Germany in December, grounding aircraft and stranding passengers.

Spokesman Ronald Schmid said that dragging Niki into the German insolvency process was wrong, and that passengers stood a better chance of getting their money back in the Austrian courts.

“We want to ensure that the insolvency is carried out in Austria – where it belongs – so that there are no conflicts of interest,” Schmid told Reuters.

IAG declined to comment.

In appointing Floether to liquidate Air Berlin in December, a German insolvency judge was of the opinion that, although Niki was registered as a company in Austria, its centre of main interest was in Germany.