When you have found a player you want to buy, offer
as much as you can for the player or enough for the
club to accept your offer. When it comes to offering
the player a contract, click at the very top of the
screen where you choose whether he is indespensible
to the squad or used in rotation etc. Click at the
top of that screen where it says the player's name
and then go to the top corner and withdraw your bid
for him and straight away approach to buy him again
for £0 or however much you want to spend. Then press
either back or continue game and it will go back to
the offer contract screen and you then just carry on
as normal and you get a player for free!