No you can't bid more money than you have available, but you can end up in debt temporarily after a bid. For example, you have a player on sale and someone bid $100.000 on that player, and you currently have $50.000 cash on hand. You are allowed to buy a player for up to $150.000, leaving you $100.000 in debt until you've actually sold your player. Note that his is only possible if there's a bid on your player that's on sale.