Microsoft warned the board of Yahoo on Saturday that if a merger agreement was not completed in the next three weeks, Microsoft would make its offer directly to Yahoo shareholders, probably at a lower price. The warning was made by Microsoft’s chief executive, Steven A. Ballmer, in a letter sent by e-mail. It expressed dismay at Yahoo’s refusal to enter into formal negotiations over Microsoft’s Jan. 31 takeover bid and warned that without an agreement by the deadline, Microsoft would seek to oust Yahoo’s board. “If we have not concluded an agreement within the next three weeks, we will be compelled to take our case directly to your shareholders, including the initiation of a proxy contest to elect an alternative slate of directors for the Yahoo board,” Mr. Ballmer wrote. “If we are forced to take an offer directly to your shareholders, that action will have an undesirable impact on the value of your company from our perspective, which will be reflected in the terms of our proposal.” Ya...