Time value of money, which in short summarized as "money available at the present time is worth more than the same amount in the future due to its potential earning capacity." [0]
[0] http://www.investopedia.com/terms/t/timevalueofmoney.asp
[0] http://www.investopedia.com/terms/t/timevalueofmoney.asp
But in any case: It comes in addition to a salary if they hire him, whether or not the $2m would be paid up front or in tranches, and if they genuinely wanted the product, not to hire him, he'd expect to be able to still earn a salary. In fact, I'd argue that he'd expect to be able to increase his income potential: He'd have a Google acquisition to put on his resume.