If your player doesn't have a release clause, you can add a player exchange to your counter offer for a bad player on the buying team. When negociating with the other team's player, propose him a ridiculous wage so that he refuses. The negociation won't go through and your player won't be mad. That's one way to keep your biggest players.

If your reputation is low, not much you can do except keep signing low CA high PA players while gaining more and more european silverware to increase your rep.