I think you and your SA not really communicate much when u bought the bag.
All of white bag or white area doesn't matter what brand seem to have that problem if you not taking care of the bag well or u accidently don't know how to take care of it.
Did you leave the bag in any moise area?
Did you leave it near by the sunlight too much?
Did you kept it in the black Chanel cotton bag?
Sometime the glue in the bag can acting up with your environment
you can tell by it turn yellow.
In your case I think your leather from black color transfer to white
due to room temp. (since black color are vegetable dye it could transfer)
Every handbags you collected should be in dry and cool area.
Egv. some people who live near by the beach their lambskin and gold hardware turns tarnish so easy and quickly.
Normally Chanel stands behind their product for 1 year.
Please do not use any leather conditioner to clean your bag it might
get worse.
Chanel is so good when it come to warrantee, I hope your SA will
help you.
If worse come to worse did you know that all the Chanel bag if they
over 1 year warrantee time and your bag is ruin or scratch, chain broke off blah blah blah. You could send back to MFG to redone your leather,chain,foots etc if they have to take out your old leather and put in the new one they can do it for less than 300.00 and it will come back just like the brand new one.
Buying Chanel is a great investment I will never sweat over that.