Spyker chief executive Victor Muller held talks with Stefan Löfven, head of the influential Swedish union IF Metall, the chairman of Saab's IF Metall branch Paul Åkerlund and the head of Saab's Unionen branch Anette Hellgren, Swedish news agency TT reported.
"We met with him for about a half-hour and he explained his thoughts about the business plan, including the financing, and how (Spyker) views production and development in Sweden," Löfven said.
"It was a good conversation and it is clear to us that this is a very serious buyer," he added.
Spyker renewed its bid for loss-making Saab on Sunday, two days after GM broke off talks with the Dutch group and said it would begin winding down the iconic Swedish brand.
Representatives from GM and Spyker held fresh talks on Spyker's new bid on Monday in Stockholm.
Swedish press reports suggested that GM agreed to resume the negotiations after the Dutch group addressed GM's hesitations over Spyker's Russian investors.
GM was also reportedly concerned about letting technical know-how go to Russia.
Newspaper Svenska Dagbladet reported that Spyker's main investors, with a 29.3 percent stake in the company, Vladimir and Alexander Antonov, were no longer behind the Dutch group's bid for Saab.
Nor was Spyker's second-biggest investor, Abu Dhabi state investment fund Mubadala Development, which owns 22.7 percent, involved in the bid, according to the report.
Instead, an unidentified Dutch billionaire was putting up the money, the paper said.
"Several sources close to the talks refer to a Dutch billionaire," Svenska Dagbladet said.
Löfven said Muller had discussed Spyker's financing in Tuesday's talks, but the union leader would not disclose details.
"They have, after GM broke off the talks last week, revised the parts (of the bid) that GM had questions about. With that done, one can say that the financing issue is resolved to the fullest," Löfven said.
"Now it is between GM and Spyker," he added.
GM has refused to comment on Spyker's renewed bid.
Swedish media have also reported that in Spyker's new offer the company has said it does not need a European Investment Bank loan before the end of the year in order to pursue its bid.
GM has said it wants to sell Saab before the end of the year.
Löfven said he considered Spyker a good buyer for Saab.
"Yes, with the financing that exists. They want to keep production and development in Sweden with Swedish management, and the business plan is the same as that hammered out by Koenigsegg and Saab," he said.
Swedish luxury sportscar maker Koenigsegg withdrew a bid for Saab in November after months of wrangling, citing costly delays in the transaction.
Saab, which has barely turned a profit in two decades under GM management, employs about 3,400 people in Sweden. Media reports suggest a Saab closure could lead to more than 8,000 job losses, including subcontractors and others dependent on the car maker.
The Swedish centre-right government, which has repeatedly said it will not take a stake in Saab, is preparing for a Saab closure in case the Spyker bid fails.
On Monday, Enterprise Minister Maud Olofsson vowed to help Saab's employees and its hometown of Trollhättan in southwestern Sweden, home to 55,000 people, if thousands were to face unemployment.
Among other measures it announced public funding of 542 million kronor ($75
million) to create jobs and growth in the region.