They will not allow you to exchange if you use the item. However, if the "faulty" issue strongly involved on the item, they will first try to get the item fixed. If exchange is allowed, they will need to check the item closely and approval from store manager. Not a comfortable issue here. If they know u r regular buyer or good customer, sometimes they do go extreme to track down the item and replace the item even it is used already. It happened to me. My case went more complicated as the item was bought in HK by my family members and the item was not in store anymore. The manager and SA went track down the whole Australia for the very last item available. They succeeded and my item exchanged to a new one. So, it really depends. Good customer relationship is important.
what do you mean by polka dots on the strap? do you mean rain spots? in that case they will not exchange as its not faulty workmanship. You should spray your bag with leather spray to prevent dirt and water marks.
I dunno what you mean by polka dots....
Even if you use it once or not at all, there are indeed PORES in the leather which may be what you're referring to as 'polka dots'. In this case, it's perfectly normal because some parts of the leather are taken from diff parts of the animal. C comme ca (like this)???
If it's a known problem they'll be more likely to exchange it (as they did with the bleeding red lining on the Damier Speedys) but if it's been used after a month, it's not really up to them to exchange it anymore. Because in that case, it wouldn't be in saleable condition anymore...if you didn't like the way it looked, chances are no one else will either.
If it has not held up with less than a months use do to being defective or the such, I think they will let you exchange it. I had a problem with the chocolate leather on the strap of a Damier Tango start peeling after a month of use, they let me exchange it no problem.