Software maker Microsoft asked search engine operator Yahoo to re-enter formal negotiations for an acquisition that could be worth USD 50 billion, the New York Post reported on Friday. Microsoft is feeling increasing pressure to compete with Google, which plans to beef up its portfolio with a USD 3.1 billion buy of online advertising company DoubleClick. Earlier this week, Yahoo said it would buy 80 percent of advertising exchange Right Media for USD 680 million, increasing its stake in that company to full control. Microsoft currently trails both Yahoo and Google in the lucrative and growing business of Web search. Update:. The deal's off.