this has happened to me with my rouge vif courier, definitely authentic. this is just how it is if you wear a bag hard... it rubs across your clothes and after some time the friction from the clothing wears down the shiny surface of the leather and there's nothing to protect the dye. i haven't gotten any hugely noticeable stains on my clothing, usually because i wear darker clothes, but i have noticed a minor colour cast on one of my lighter-coloured items.
this really sucks, but balenciaga leather is unfinished, so i guess this is just the way it goes. unfinished in the sense that it doesn't have a tough protective coating over it of some sort...
how often are you using this bag and are you carrying a lot in it? this has only happened with my red courier, which i carry a lot of stuff in, at least 5 lbs. on average, so that surely contributes to the wear.