Well let's say you buy player X for £1M with '50% of next fee' activated.  If you sell him for £2M, the club you bought him from will get £1M.  If you do the same but with '50% of profit from next sale' clause activated, and sell for £2M again.  You only give away £500K as that is 50% of the £1M profit which you made on that player.  I'm not sure however, what would happen if you had the 'profit from next sale' clause and then made a loss on him.  Hope this helps! 
P.S. If you don't sell him again - i.e.e you release him or he retires, you don't gave his previous club nada.