GM and PSA will have to work out pension plan details if Opel is to be sold.

General Motors' sale of its European Opel division to France' PSA Group hinges on funding for the automaker's retiree pension plan, a new report finds.

Although both GM and PSA seem eager to get a deal done for the Opel brand, an underfunded pension plan looms large in the negotiations. Bloomberg estimates that the retirement pension plan for Opel and Vauxhall employees is underfunded by about $9 billion.

Naturally, neither side wants to be on the hook for the full amount. PSA has offered $2 billion for the Opel brand which includes $1 billion in cash and another $1 billion to be used toward the pension plan, but GM doesn't want to be responsible for such a large investment in a company it no longer owns.

However, there is incentive for both sides to give a little ground. GM spent about $1.2 billion to keep Opel afloat last year, so there's savings to be had through the sale of the division. On the other side of the coin, PSA believes it could save $1 billion per year through joint development with Opel.

Negotiations are ongoing, with both sides hoping to reach a deal in the very near future.