when a club negotiates the transfer, it usually means your previous bid was close to the minimum value that they would accept. on fm every player has his price, and once the selling club starts negotiating you are usually very close to matching it.
as an example, i recently bid for pirlo. my starting bid was £13mil, and AC negotiated back asking for 24. i then made another bid at 16 and it was accepted. a mistake lots of players make is that they think that the negotiated price is the minimum they will ever get that player for, however if they continued negotiating they would find that its usually a lot lower than they think. its all about patience.
another tip is to unsettle the player, most people do this anyway but i find that the selling price sometimes goes down dramatically. i once had a 20mil bid for a player rejected, then after i unsettled him my cheeky bid of 14mil was accepted.
thats all i can think of, hope it helped