Easiest way to make big and quick money is to sign young Free Agents with high OVR rating. To sign them you have to pay them a high per game salary, do whatever is necessary, then once u buy them, put them up for sale, you'll make millions real quick.
Also sell players you don't use. Or loan them out.
Not in transfer season? How'd u let it get to that? Besides raising ticket prices to high if you haven't already, nothing you can do. Unless you have money to increase your stadium size to bring in more ticket revenue, you will have to wait it out until the 2nd transfer period and either do what i recommended or sell a high salaried player quick.
I believe if you end up in the hole you get fired as manager.
"wait it out until the 2nd transfer period and either do what i recommended or sell a high salaried player quick."
Negotiations for a free agent could take 3 or 4 games depending on how high your negotiator is, so if u need quick money and can't wait the 4 games into the transfer period, sell off a high salaried player, and he'll be sold by the second game into the transfer period.