Not quite sure I understood, but you want to have null values when the value of your Einstandspreis is zero?
If so, you can use a conditional on your load statement, like this:
if (num# (Einstandspreis, '#.#', '.', ',')>0,num# (Einstandspreis, '#.#', '.', ','),null() as Einstandspreis
If your number interpretation is greate than zero, it will input null as "value".
If I misunderstood your problem, please add more to it and i'll try to help.