Once you receive an offer for a player press respond.
Then up the value to anything you like upto the 150GBP max.
Then press home button in top left corner.
Then press mail button in top left corner.
Then press respond to go back to offer page.
Then press home button again.
Then press back button to take you back to the offer page again.
Then accept the offer.
Once done it will say you have accepted the offer for the original value the other team offered but when you come to complete the offer it will pay you 150m or what ever other amount you asked for regardless of if that club would have that money to spend.
Great way to make you own team of player from any league in the game.