I just did a bg80 at 24lbs string job on a racket and when the owner came to collect it he noticed a paint chip on the racket, inside of the frame, around the 8th hole/main ending knot.
I did not notice the damage on getting the racket and do not have any recollection of chipping it while stringing (do not use string movers etc. any more, just cutting the string in a point and I always get it through).
Owner says it must have been done during stringing as the racket was 'new'. It had been strung by the store when he bought it, so no factory string.
if he's so pedantic, he should have warned u before the job... I would've declined any refunds. But if you think that he is a promising customer and will come back (which i doubt), you can meet him half-ways and charge for the string only. But 10 euros is already a very cheap stringing, the BG80 in 10-meter pack costs more than a half of that...
ps I wonder, if he had a clash with his doubles partner, would he bill him for the chip? ))