The deal involves Berkshire (BRK.A)(BRK.B) being a 50/50 equity partner with 3G. Berkshire will also own some preferred stock that 3G won't. In essence, Berkshire is both a financing and equity partner.
Buffett is willing to pay a 20% premium to the recent share price because the company has a great group of products and clearly has a moat that is impossible to scale.
Here is Buffett discussing it: