Set "fill" and "stroke" opacity independently in inkscape

Yes, there is such a possibility. Every color within Inkscape has four components:

• R ed
• G reen
• B lue
• A lpha

Alpha is basically that transparency of this color and this color alone. While the Opacity is Object-wide.

The difference between transparency of one color and overall opacity is that parts of an object can shine through other parts of the same object. The second row in the picture displays this nicely, as the background shines through the border. This is not the case if you set the overall opacity.

Anyway, you're looking for the fourth slider in the Object-Properties dialog which is labeled "A".

And sorry for the german interface...