"Buy" off the free agent list.you dont actually have to pay a transfer fee.You just pay him really really high wages.You can then keep the player(if you can afford his weekly pay) or sell him straght away for a huge profit.

I was playing as Sydney FC(low budget), so I "bought" 3 guys off the free agent list.(You do have to pay their wages for at least a week or until you can sell them)I then sold these 3 guys for about $10 or $15 million each.

This will work with any club on the game, even the rich ones like Real Madrid or Man United.