Asked whether the deal could collapse, Denis Bugrov, a member of Sberbank's management board, said he "did not exclude the possibility."
"Anything could happen," Bugrov said at Sberbank's annual shareholders meeting.
German officials have stressed that the deal is preliminary and have said that until a final agreement is nailed down, other potential suitors could at least theoretically still be in play.
Sberbank expects a final deal to be signed no earlier than September in the most optimistic scenario, Sberbank chief German Gref said Friday.
Magna and Sberbank are expected to agree by July 15 whether to push ahead with the deal to buy Opel from General Motors after signing a nonbinding agreement last month, which envisions Sberbank taking a 35 percent stake and Magna a 20 percent stake.
The deal was seen as a major coup for Russia's troubled carmaking industry based on signals that Magna would produce Opel cars in Russia.
On Friday, Oleg Deripaska's Basic Element holding said carmaker GAZ was training workers at GM's Opel plants n Germany and Austria. The program is part of the quality-management system of Opel's that GAZ has adopted, Basic Element said in a statement.
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