Billionaire investor Carl Icahn again took to Twitter to chat up Apple — just more than a week after he tweeted he has “a large position” in the computer giant, believes it to be “extremely undervalued” and spoke with CEO Tim Cook about it all. Today, he tweeted: “Spoke to Tim. Planning dinner in September. Tim believes in buyback and is doing one. What will be discussed is magnitude.” Icahn last week had told Reuters that Apple should repurchase $150 billion worth of its shares after borrowing the funds at an interest rate of 3%, saying the stock should trade at $700 if earnings increase as little as 10% and his advice was followed. Last week’s tweet helped boost Apple stock — down 23.4% over the last 12 months — by 5%; today Apple is mostly flat and is trading at $502.73 a share, up about 13 points since Icahn confirmed his stake rumored to be worth about $1 billion. In March, Apple announced plans to buy $100B of its shares by the end of 2015, calling it the largest share buyback of any company in history. At the time, it raised its dividend 15% to $3.05 a share. Of course famed corporate raider Icahn has played less nice with media-centric companies before including Blockbuster Video and more famously Lionsgate.