You only need to have enought money in the first bid, after that you can end with negative money.

For exemple you have 20M and you put a bid in 2 players of 15M each.
You won the first one and you stay with 5M in your account, since you already put a bid in the second player, you can continue to bid.
You have 7 days to have positive balance in your account, otherwise your highest value player Will be put for sale.