what happens if u give the 15000 gp to valens mistress instead the shadow thieves?
.......... if it happens anything at all...

I'm assuming you mean where you the offer is proposed in the graveyard...? If so, it's another way to progress through the game. It has been argued that paying the Shadow Thieves is the "good" way to progress, while the other way is more "evil." Make up your own opinion, but both ways should get you were you need to go.