He may have bid when he had enough money but bid on other players at the same time and won a player while the pictured auction was still live. It doesn't cancel your bid.

For example
You bid on player A worth 1M
You also bid on player B worth 1M
but you only have 1M to spend but you win both auctions

You will go minus/overdrawn 1M which has to be paid back within 7 days or the game sells of players to recoup the money