Frederik,
This appears not to be an uncommon problem. I had a local sailmaker sew a new sail for my Johanna, back in 2002. He screwed up completely and had to do it all over again. It surprises me that the Tuchwerkstatt makes such errors, though, since Sebastian has made so many junk sails. Could the job have been left to an apprentice without sufficient supervision?
Anyway, this is why I encourage people to make their own sails, and that is why I still ‘sell’ the simple barrel cut method: This method lowers the step from the armchair to the sewing machine.
I claim that anyone who has never ‘set foot in’ a sewing machine can gain the needed skills in two days, with a bit help. During those two days one learns to adjust the thread tension, how to start and stop (!), and by making a couple of sail bags or shopping bags, one soon learns to make the seams straight enough. I know about no other (legal) homework, which pays back better than making my own sails.
The sail can probably still be used (good that you made the mast a little taller), but you should have a serious talk with Sebastian. He cannot be proud of that job.
Arne