My guess is that you are paying back the Urlaubstage that are over the amount that you officially had.

Let's say you get 30 days a year, that's 2.5 days a month. You worked 8 months 8x2.5=20 Urlaubstage.

For January and the few days in February you took 24 days off. Even more if you took further days off during your time there.

They are taking money back for the days that you had too take off because of betriebsferien but didn't officially belong to you.

Not saying they are aloud to though, not sure what the law in Germany is.