The brass CAN dirty the leather, the best you can do is make sure you clean the brass peices regularly to stop them from transferring black to your leather, also always undo any buckles/locks etc when the bag is not in use and store everything separately in the dustbag. My denim baggy has already got buckle marks and its pretty new. Its always best to seal your leather with some kind of spray like appleguarde, I don't think it would STOP the black transferring from the brass but it would certainly slow it down.