Rakuten share prices fell further Monday than they had in four months after the owner of Japan's largest online mall said it is in talks to buy U.S. website operator Ebates Shopping.com Inc.

Rakuten, controlled by billionaire Hiroshi Mikitani, is negotiating the acquisition, according to a company statement released Saturday.

Tokyo-based Rakuten may pay about ¥100 billion ($950 million) for Ebates and is nearing a final agreement in a deal that is expected to be announced as early as this week, a source familiar with the matter said, asking not to be named as details of the talks are private.