Reduce window area - not the opening

how could such deviations occur? I also think that reworking the windows would be too expensive. Then rather enlarge the window openings. That is actually quite easy downwards, sideways it gets more difficult, but still possible. In any case, it costs money. In our renovation, we adjusted almost half of the windows to a different size and also installed some where there was no opening before. But we also have a timber construction for about half, which is really child's play (there are no lintels, after all).

The question of who is to blame does not arise.

You wrote that you "are not in the mood for a legal dispute"...? You cannot have the windows reduced in size; the costs – if at all possible – would be about the same as new windows. If the wall opening is widened, the lintel must also be enlarged; this would not have been a problem with "only" deeper windows, which is why my first thought tended in that direction.

I am looking for a way to limit the damage.

Spontaneously, I would suggest checking out exchange platforms; maybe there is someone around who is missing your size. Otherwise, sell and reorder new ones; the first pain is always the smallest. Rhineland greetings from on the road