As far as I know there are no cheats for fifa 12 but you can still buy top quality players without actually cheating:
Go to Buy Players and search for whoever you want to buy and select that player
Press across so you are on the screen to make a loan bid
At the bottom there is a future fee option. Even though you have no transfer budget you can put whatever future fee you like and you don't need to increase the players wages either. (I.e. You get the player on loan for the season and only pay his wages) If you haven't got enough wages go to budget allocation and adjust your budgets.
The players club will reject the offer if the fee is too low ,if the player has recently joined them or if the club is short on players.
If your offer is acceptable you will recieve an email from the players club stating they believe the fee is fair but they are unsure whether they want to send the player out on loan and then sell him. They will then send you another email a couple of days later accepting your offer.
At the end of the season you can either pay the fee (if the board gives you enough accomplishment money) or send him back to his club and loan him again when you start the next season.