Why is _no one allowed_ to install my washbasins?

If the general contractor calculates a washbasin at €500 and credits you €200 for deducting it, he has also made his cut

I have the plumbing list with the exact products. On site, the prices are displayed on the signs... So honestly: If he should only deduct half for me, then I would never build with such a general contractor.

He might as well completely deny everything. But that would be a total rip-off...

You can try googling "Bundeskartellamt Sanitärgroßhandel"

What you can do is tell the general contractor that you have specific ideas about the bathroom furniture and ask if there is a possibility to obtain it yourself if he cannot get it through his wholesaler. But it really has to be something special, like a designer bathtub. Mine accepted this for a lead crystal washbasin.

Then there will be exactly no general contractor left who will build you a house

Then there won't be a single GU left to build you a house

If he doesn't do it - then so be it. It's worth a try to maybe get a few more percentage points elsewhere.

But only calculating half for me? Come on, that would be total rip-off

We do not know his calculation for heating/sanitary, with some luck there is a total amount listed, but it goes in this direction