I would not consider a scratch to be an "acceptable manufacturing defect".
The 1955 edition of Morgan & Lester's "Leica Manual", in the section on Lenses, they go on quite a bit about some lenses having tiny bubbles (air inclusions), and how this was considered "allowable" by Leitz and other mfrs... they go into some granular discussion about the size of these bubbles , relative to the size of the element, etc... and explain how they should have no discernable effect on the image.
(I recently picked-up an 85 mm Zeiss Opton Sonnar (Contax RF) for a friend. The lens is in excellent cosmetic condition, and the glass is free from scratches & cleaning-marks; but I was quite surprised at the presence of several tiny bubbles in the lens elements; even from the almighty house of Zeiss. my friend was thrilled with the lens, and when I pointed -out the bubbles, he acknowledged them, but wasn't too worried about them. The lens takes fine pictures.)
Bubbles in glass are most definitely a manufacturing abberation; a scratch is the result of careless handling, on somebody's part, whether in the factory, or somewhere along the way.
If the dealer values his customers, he should not have a problem taking the lens back, then insisting that the lens manufacturer make things right with him.
At least, that's my philosophical point of view....
I don't buy new lenses because I feel that I can't afford them; and if I did spring for a new lens, it would be because I'm weary of trying to find thread-mount Leica lenses that aren't scratched and foggy, so that new lens had better be in perfect condition. ( and I'd prefer it not have bubbles ! )
Good luck !
Luddite Frank