-Make a bid for the player with a cash price, and no exchanges or money after appearances. Once the bid is accepted, click on the player's name at the contract stage. Then, go to transfers, click on your bid, and withdraw it. Immediately make a fresh bid for £0 and offer it. Then, press "Back" to return to the contract stage. Offer a contract as normal and if the player accepts it, you will get him at no cost.
For example, to get Ronaldo free, add manager as Inter Milan, release Ronaldo on a free transfer, and retire from the club before they cancel the transfer. Go another club and sign him for free.