GU demands a price increase on the entire house

I would not assume that at all. Building material costs a fortune.
My general contractor recently told me that the only reason he can still offer fixed lump-sum prices and adhere to them is that he has the necessary liquidity to advance payments.

Well, if he has to advance payments with his liquid funds, then he will surely get it back from the end customer after the work is completed. That’s what this is all about. Then I don’t need a fixed price; he should just present the invoice at the end and can sue me for the money if the customer doesn’t want to pay for almost two houses ;-) just turning the situation around

If the OP had taken that into account, he could now calmly fall back on the WYSIWYG standpoint, consider the achieved construction status as "his," and assign the further completion at his own discretion without fearing protest from the general contractor.

Could have, would have, chain. The OP has already admitted that he acted naively when signing the contract. I merely said: From the point of view of the situation he is in today, an amicable agreement and the tuition fee after the settlement is probably the best he can get. And sometimes you can just be satisfied with a status quo. Lay down a bit of your Germanness, let the "Alman" be silent, and just accept it when it's okay because it could have been worse. ;) Because what would his alternative be currently? We have already extensively discussed beforehand that legally he is on thin ice, and thin ice in court is dangerous and expensive and so on and so forth.

I simply said: From the perspective of the situation he is currently in, an amicable agreement and the lesson learned after the agreement is probably the best he can get.

At least a pretty reasonable option, yes.

Let’s drop the Germanness a bit, let the Alman be silent,

*ROTFL* I can’t even count how many times in business life I have seen a Muslim let the Alman show through when that position was closer to his advantage :)

Because what would his alternative be currently? We have already extensively discussed beforehand that he is on thin ice legally, and thin ice in court is dangerous and expensive and so on.

He has largely cut off alternatives for himself beforehand. I didn’t write my most recent post least of all for the readers who are still at the “at the bike chain” stage and can avoid repeating the mistake. The builder’s thin ice here lies at the point where the unfinished construction can still suffer (weather) damage. In court, it is rather the main contractor’s ice that is thin here, wishing to present a claim without a comprehensible calculation.

In court, the GU’s position is rather precarious to present his demand without a comprehensible calculation.

I am sure the GU would come up with something in court if it came to that. Just because he doesn’t want to disclose his calculation without pressure doesn’t mean he has nothing up his sleeve – or that neither he nor his lawyer can think of anything. There is also the old saying "in court and on the high seas...", about having the right and getting the right, even if we assume that the client is completely in the right.

I also find it dangerous to assume that the contractor just threw out the price randomly. Especially since we don’t even know which contractor it is. I suspect that the price of 275,000 EUR before adjustment is something like Town & Country or similar.

Those prices come from above and do have a basis. I certainly don’t think it’s a given that you would win a process here.

And somehow this viewpoint annoys me, that the entrepreneur could just give up a few euros of profit... that’s a naive perspective, I want to receive my salary monthly, in full, and not give up part of it because the customer doesn’t want to pay the full price.

I’m sure the general contractor would come up with something in court if push came to shove. Just because he has no desire to disclose his calculation without pressure doesn’t mean he has nothing up his sleeve – or that neither he nor his lawyer can think of anything.

He can’t choose that. No one other than him is requesting the price adjustment here (under §313 of the Construction Code or what other basis should he cite?), so the burden of proof for the increased costs lies with him. If he doesn’t want to show his cards, he can only withdraw his price increase request (or choose the option of terminating the construction contract – but he must also prove the disruption of the basis of the transaction for that and not just claim it). The TE’s risk here is that the general contractor opts for termination – but in that case, I believe the TE is fine since, if I recall correctly, he has not presented the difference between the actual value of the achieved construction progress and its tentative reflection in the payment plan as significant so far and would be willing to continue to fend for himself. Therefore, his risk would be zero as soon as the general contractor declares the intention to choose the termination option. A deadline can be set for the general contractor, and if it passes without success, the TE’s termination under §648 of the Construction Code can be announced. I see nothing in this matter that depends on the ingenuity of a lawyer or the judge’s mood. The Construction Code has pleasantly short paragraphs in this area ;-)

And somehow this viewpoint that the contractor could just afford to forgo a few euros of profit annoys me… that’s a naïve perspective, I want my salary monthly, in full, and not give up part of it because the customer doesn’t want to pay the full price.

The customer is paying the full price, just not any fuller. And the contractor is free to choose whether to play the "proven increase" event card or the "termination" event card under §313 of the Construction Code if he doesn’t want to use anything from his margin to cover a price increase risk exceeding expectations.

Both parties made mistakes, but both can also come to an agreement or part ways. A construction contract was not made before God; humans may dissolve it :)