Niki's German administrator Lucas Floether and his Austrian counterpart Ulla Reisch said in a joint statement that they would co-operate closely on sealing a deal on the future of the insolvent airline.
Floether said as recently as Friday that he was considering legal action against an Austrian court ruling that Niki's main insolvency proceedings had to move to Austria from Germany.
This came after Floether had already agreed Niki's sale to Aer Lingus and British Airways owner IAG.
"The signatures of both administrators will guarantee the buyer legal security for the closing of the sales contract," Floether and Reisch said.
They reiterated that bidders from previous rounds could issue fresh offers until Friday.
Meanwhile, Ryanair said it has contacted the Austrian administrator of insolvent holiday airline Niki to express its interest in buying some of its assets.
Former Formula 1 world champion Niki Lauda has also re-emerged as a potential bidder for Niki ahead of a January 19 deadline for fresh offers the Austrian court has set.
Niki, a unit of collapsed German carrier Air Berlin, filed for insolvency in Berlin in December after Lufthansa scrapped plans to buy part of it.
After renewed talks, IAG then agreed to buy Niki, founded in Austria by former motor racing champion Niki Lauda, and make it part of its Spanish budget carrier Vueling.
In a statement, IAG said today that it remained interested in the assets of Niki and is looking forward to the new process being completed promptly.