"There was an agreement that Magna and GM on July 15 should be far enough in their negotiations that they can reach a conclusion that serves as the basis for a contract," a person familiar with the matter told Reuters.
Neither Magna nor GM Europe would comment on the time plan.
Siegfried Wolf, co-CEO of Magna, said on June 3 that he expected a "final signing" in four to five weeks, after which it would await the necessary regulatory approvals before an expected closing by the end of September.
Two days later, GM Europe President Carl-Peter Forster confirmed the Magna co-CEO's rough time plan of a "definitive agreement" by July and a closing by September.
During the time that General Motors operates under Chapter 11 bankruptcy protection and until a sale to a new investor is concluded, 65 per cent of Opel's shares are formally held by a trust set up to ensure that a 1.5 billion euro (US$2.08 billion) bridge loan extended by Germany is not misappropriated.
Rival bidders waiting in the wings such as Fiat S.p.A. and Beijing Automotive Industry Corporation (BAIC) are still hoping for a collapse in the talks in order to re-enter negotiations with GM.
A source familiar with the bidding process said on Tuesday that BAIC received approval to advance one stage further, gaining access to financial information over Opel that Fiat and Magna previously had.
BAIC had already named PricewaterhouseCoopers as financial advisers earlier this month.
GM and Magna have already largely resolved issues such as the licensing fees Opel will pay for access to GM technology.
Other sticking points, such as where Opel is allowed to sell cars or who ultimately bears the risk for its 4 billion euros in pensions should Opel file for insolvency, have yet to be resolved.