By Alexei Oreskovic SAN FRANCISCO (Reuters) – Yahoo Inc pledged to pay its shareholders at least half the proceeds from Alibaba Group Holding Ltds mega-IPO this fall, and plans to keep a larger stake in the Chinese e-commerce company than expected. Yahoos roughly 24 percent stake in the worlds largest Internet retailer is viewed on Wall Street as its most prized asset. On Tuesday, it said Alibaba had agreed to let Yahoo sell at most 140 million shares in the IPO, down from a previous cap of 208 million. Yahoo also forecast third-quarter net revenue, excluding fees paid to partner websites, of $1.02 billion to $1.06 billion, less than the $1.1 billion Wall Street analysts had expected on average.