Errror: double stock calculation when using custom field (variant)

[hatschi-don]

Hello,
I configured a product having differnet prices depending on variants by using custom field (variant).
Display of the parameter and price calculation works pretty good.
However, the stock calculation after the status changed to pending and finally to shipped does not work correctly. Each item is counted twice, that means that the order of 1 article results in 2 article less in stock.
Does anybody have an idea about this?