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.