Renew windows / Is the offer fixed or negotiable?

The windows were installed during the house construction by a local master carpenter who still exists today (in the second generation).

That leaves open whether they are wooden windows ...

There is no alternative for the property because only the windows on the first floor are being replaced and accordingly the muntins on the ground floor remain. To maintain the overall visual impression, the muntins have to be put back in.

... if they are wooden, plastic would already look noticeably mismatched.

That leaves open whether they are wooden windows ...

No, they are regular plastic windows.

Addendum: Test your window installer with the following...

DIN 1946-6 states that for all new builds or renovations where more than 1/3 of the windows on the building are replaced, or in single-family houses more than 1/3 of the roof area is sealed, a ventilation concept must be created.

DIN standards are not binding and do not have to be implemented. It only becomes interesting when the legislator takes it up and makes it mandatory.

DIN standards are not binding and do not have to be implemented.

No. However, the Building Energy Act legally states that you must comply with a minimum air exchange rate to protect health and prevent moisture damage. You can only ensure this through the recognized rules of technology. And you can only comply with these by following the DIN. In this respect, the non-binding standard again becomes a legal obligation. Because the Building Energy Act forces you to do so. If mold develops after the replacement, this chain becomes a problem.