Apollo Tires’ $2.5 billion buyout of Cooper Tire & Rubber (CTB) looks to be on a bumpy road to completion, with the two companies engaging in a heated debate over the price of the deal.

The squabble has unnerved investors, sending shares of Cooper down 11.4% to $26.14 early Monday morning.

Apollo, an Indian tire company, agreed in June to buy Findlay, Ohio-based Cooper. The deal was expected to close by year’s end.

Labor-related issues in the U.S. and China now threaten to derail the agreement. Before closing the transaction, Apollo intends to reach a union deal in the U.S. and negotiate terms with Cooper’s Chinese partner.

As a result, Apollo said it may have to incur “significant and unanticipated costs” beyond what it was obligated to take on under the original deal. The company also suggested that Cooper has acknowledged the need for some price reduction.

But Cooper denies any concession was made, saying it “has not agreed that a reduction in share price is warranted.”

On Friday, Cooper said it filed a lawsuit in Delaware in an effort to force smaller rival Apollo to close the $35-a-share deal.

“Cooper has an obligation to protect the rights of our stockholders, who voted overwhelmingly in favor of the merger. With their approval, we have met our conditions for closing,” Cooper CEO Roy Armes said in a statement last week.

Armes added that the complaint is a “necessary step” to ensure the transaction is “closed promptly.”

The labor issues represent risks Apollo assumed under the original agreement, Cooper said in a separate statement.

Apollo is talking with the United Steelworkers, which represents Cooper workers in the U.S. An arbitrator ruled that two Cooper plants can’t be part of the buyout until a collective bargaining agreement is reached.

Workers at Cooper’s China joint venture have been on strike against the deal, and its partner there filed a lawsuit to stop the transaction. Workers have refused to make Cooper-branded tires and have stopped providing the joint venture’s operational and financial information to Cooper.

“We believe that the situation with the USW can be resolved in a timely manner and ask that Apollo proceed expeditiously toward resolution with the USW, working with Cooper,” Cooper said. “We continue to work toward resolving the issues in China through communication with the workers, union and the joint venture partner.”

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