Quantum Group's 7.5 billion euro ($9.1 billion) bid for Volkswagen Group's supercar brand Lamborghini still stands, the Swiss group's founding partner said, days after VW said the unit was not up for sale, Automotive News reported.

"Of course our offer is still valid. After all we have carefully weighed and thought through our decision for Lamborghini and the attached offer and concept," Rea Stark told Reuters.

"And if there is willingness to negotiate -- whether that's investments, guarantees or the purchase price—then of course we are still prepared," he said.

Stark said that in addition to the 7.5 billion euro bid, first submitted to VW in late February but only made public this week, Quantum has offered 1 billion to 2 billion euros to cover investments and guarantees for jobs and sites at Lamborghini.

"We deem this a fair first offer, it's a valuation that is common for luxury brands," Stark said.

Rival Ferrari currently trades at an enterprise value of 21 times EBITDA, according to Refinitiv data, compared with 2.3 times at Volkswagen.

Last year, sales at Lamborghini fell 11 percent to 1.61 billion euros. The company does not break out core profit (EBITDA).

The offer drew a cool response from VW, which earlier this week said there was agreement that Lamborghini, repeatedly named as a possible divestment candidate, will remain part of the group.

Audi, which is in charge of Lamborghini, said the unit was not for sale.

"The message from Volkswagen and Audi was clear. One has to accept that," Stark said.

Stark is also co-founder of Piech Automobile with Toni Piech, the son of former Volkswagen Chairman Ferdinand Piech.