Oasis Management Co.'s Seth Fischer has increased his pressure on Panasonic Corp. to sweeten its offer for listed subsidiary PanaHome Corp., calling the transaction a test case of whether Japan's corporate governance overhaul is working.

Oasis will ask the courts to rule on whether the price is fair if shareholders approve the deal on current terms, Fischer said in an interview in Tokyo last week. The Hong Kong-based fund, PanaHome's second-largest shareholder after Panasonic, says the offer undervalues the homebuilder by more than 50 percent. Fischer's calling for a bigger share-exchange ratio or for PanaHome to pay out its cash as a special dividend.

"They're basically taking our holdings at a cheap price," said Fischer, Oasis's chief investment officer. "It's just not right."