Cart Variant tax issue

[jjk]

J1.7.3/VM2 SVN revision 5163
When using a Custom Field - Cart Variant, all taxes (i.e. in Germany MwSt. 19%, MwSt. 7%, Differenz-Steuersatz) are added simultaniously to the "Cart Variant's additional price in the Product Details view. In the cart itself it just adds the original additional value typed into the custom field price. Any idea how to solve this?