It is rather confusing, but here's how that happens:

Let's say he had 22M to start with, and a quantity of Tokens. He found a couple players he liked, one at 14M and and he put a Token on him, and the player you saw him on, he can also put a Token on... because he at that moment had 22M. But, he got the first player before bidding against you, so he then was already qualified to bid, having put the Token on earlier, but he would wind up with negative balance if he won the player. The bank will start repossessing players starting with most expensive if his balance isn't fixed in a week.