This can happen, example, you put more than 1 bid on player, lets say you have 25M on your bank, and you put 2 bids on player @20M ea, since player price lower than your balance then you can put bid on both player, and if you win both player then your balance will minus, 25M - 20M - 20M = minus 15M, and you must clear your minus in 7 days before system sell your highest price player to cover your debt.

Another scenario, you only bid 1 player, but stuck in bidding war which increase player price, your balance will minus if player price > your balance after you win that war.

If you balance lower than player price then you can't put bid.

As long as your balance higher than player price then you can put many bid on player, your balance minus or not after auction end then depend on how many player you won, you still take that player but your balance will minus.

CMIIW...