The Luton plant has a contract to build Vivaro vans as part of a deal with Renault but the French carmaker has a clause allowing it to pull out of the contract if there is a change of ownership.
Some observers have criticised the UK government, arguing that it has been outmanoeuvred by its German counterpart, because it has received specific commitments about safeguarding German jobs.
Magna has said it will not close any plants in Germany.
This pledge meant that the German government put pressure on GM to sell Opel and Vauxhall to Magna. It also provided a 1.5bn euro ($2.2bn; £1.3bn) bridging loan to keep Opel operating in recent months.
Last month, it also said it would lend Opel 4.5bn euros if the carmaker was sold to Magna.
Lord Mandelson said there was "British money on the table" should the company need financial support.
"I have had four face-to-face meetings with Magna in the last couple of months. I am satisfied about their commitment to Vauxhall's plants," he added.
"The government will now discuss our share of the funding of the deal and I expect to tie down the details of this in the near future."
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