The revised bid leaves the total proposed amount at some 500 million euros (US$714 million), but tilts the financing mix towards an immediate equity injection instead of using convertible debt for the bulk of the funding, Reuters said.
In a blog entry posted on the company's website on Tuesday, GM's chief negotiator said he still expected the deal to close by the end of September, although no preference as yet has been made for a specific bidder and key points had yet to be resolved.
"The bid from RHJ International is completed, and would represent a much simpler structure and would be easier to implement...This represents a reasonable and viable option to be considered as the very difficult issues around the Magna negotiations continue to be worked (on)," GM's John Smith wrote.
GM has previously indicated a preference for the RHJ offer as there is the possibility that the Belgium-based company would eventually sell its interest back to GM, something the German government strongly opposes.
Magna's new offer is expected to silence some of the critics in the German government that have admonished it for effectively shifting all of the risk onto German taxpayers, who will largely finance the deal through billions in loan guarantees.
Germany's economics minister, Karl-Theodor zu Guttenberg, has said that European cartel authorities would look at Opel's ratio of equity to debt as a key criteria for whether a new owner just served as a fig leaf for a state-sponsored bailout.
"Magna is now offering 350 million euros of its own capital immediately," said the government source, who is familiar with talks to find an investor for GM Europe, which builds Opel and Vauxhall vehicles.
"Furthermore, there should be a 150 million euro convertible bond."
The Canadian company and its consortium partner, the Russian Sberbank, are competing with RHJ International for GM Europe.
GM can no longer afford to finance its European carmaker and is being forced to give up majority control in exchange for substantial government aid for Opel/Vauxhall.
"The world is a different place with 350 million. This proves it was a correct decision to negotiate with two parties -- otherwise Magna never would have improved its offer," one source close to the talks said, adding that the only remaining hurdle for Magna now was reaching an agreement with GM.
Magna's founder and chairman, Frank Stronach, met with GM Chief Executive Fritz Henderson on Monday in Detroit to discuss some of the last issues of contention between the two.
Magna wants to expand Opel's full-scale car assembly business and forecasts high growth rates, particularly in Russia.
RHJ aims to shrink production to return Opel to profit and may be open to selling it back to GM at a later date.