Buy the rumor sell the news crap

Imagine you own a stock. Let's say it's current market cap is $1B. You hear that an investor is waiting to put another $1billion into that business - not all at once but over the next few months. He's just waiting for the funds to clear on January 10 and he'd be ready to put an undefined lump sum in as early as January 12. Would you sell the stock on January 10 once the funds clear if he's clearly about to put an amount in that basically reflects the current market cap? Even if he "only" puts in $500M, why would you sell at current market cap pricing?