When an Agent offers you a player, you don't have to make a decision straightaway on whether to accept. 

Any Agent Offers that aren't accepted or rejected will go to the 'Negotiations Room' for 12 hours, giving you time to consider what you want to do. If you decide to accept or reject a player, you can do this from there. 

After 12 hours, negotiations will break down and players will automatically be rejected. 

You can access the 'Negotiations Room' from the 'Home' screen, next to the Agent offer.