That's not true and has turned into something of an urban legend with uninformed people repeating the claim. Here is the list http://www.networkworld.com/news/2011/070611-microsoft-andro...
So I really don't buy the "B&N scared Microsoft into bribing it" story. To me, it seems more like a "B&N tried a Hail Mary shot at getting an antitrust thing going against Microsoft, and when that failed miserably, they sold a cheap stake in their ebook business" story. Consider that ebook content is something Microsoft's ecosystem didn't (and still does not) have compared to Apple, Google and Amazon's ecosystems. Consider also that this ebook content comes without the lawsuits and publisher-wrangling the others had to go through.
So, $300M for a quick entry into a market where its competitors are incumbents seems like Microsoft got a pretty decent deal, and I think that patent lawsuit provided leverage.