I tend to see this a bit differently. I would of been called in to remove soot damage from a carpet following an accident or lack of care. This IS the builders problem, NOT MINE. As long as I have not raised expectations I can only do the best job possible. Beyond that the builder or householder can make a claim on their insurance. The customer is no being irritating, they had a brand new carpet and if it was me i'd expect it to be in the same condition. I would go back and re-clean as a gesture, possibly LM, but no way would there be a refund for failing to remove damage caused by someone else, this does rely on pre-qualifying my customer.