In the aircraft.cfg, you can change the ZFW to a different amount there:

[weight_and_balance]
max_gross_weight=766800
empty_weight=318300
max_zf_weight=461000
max_landing_weight=575000

Change 461000 to the weight that Leipzig Air specifies.

Be aware that the max_zfw_weight must be in  pounds, not in kilograms!

i.e. if the ZFW is 250,000 kilograms, then you would change it to this:

[weight_and_balance]
max_gross_weight=766800
empty_weight=318300
max_zf_weight=551155
max_landing_weight=575000

Backup the aircraft.cfg BEFORE you make any changes to it!

Note. I used Google Translate on you original post, so I hope this answered your question good enough for you.