Shirt sales for one, I'm pretty sure Adidas get a cut.
The more he's in the lime light, the more people talk about him/follow him on twatter the better it is for Adidas, this deal was a no-brainer for them considering they sponsor both player and the club.
Say a shirt is £50... 400,000 shirt sales would generate £20M in total, obviously deduct the percentages ADIDAS get and it'd work out less.... United will easily sell millions of shirts with POGBA on the back and already have been doing before he even put pen to paper, so Adidas would see that £20M return easily. I'm not sure what money they make from sponsoring Pogba himself but I can imagine every time his image is used, they make a fair amount, I'm sure the publicity he's getting world wide now would rack a few pennies in?