If I were Tim I would be sympathetic to the woes of a customer--as a business owner (and fellow human) I want to be fair in all circumstances. But if this defect is detected several days after having left the store I, as an otherwise happy customer, would not be inclined to press him on it. However, also as an otherwise happy customer, I might pose the question to him as a hypothesis to see what happens--at the very least he will be aware of the problem with his shop's handling of a customer's valued merchandise. If it was within 24 hours I might feel justified to extract some compensation (but if I delayed while considering the point I would have lost the opportunity). I don't think his having performed the work for free is an issue, but we might prevail upon more experienced (anyone's but mine) or legal minds (did our moderator confess to having possessed one?) to weigh in.

Best wishes.
Robin