Costs of real estate business within the family

Why shout like that right away with all the exclamation marks?

Nonsense.

If the property is sold, the mother has €400,000 and this amount will be inherited by someone in the event of death (if it wasn't spent, let's leave that aside). If the mother only grants a loan of €400,000, the daughter also inherits these claims against the man.

Because the goal in the initial post was to minimize costs.

I find it reads as if it is still unclear how the future ownership situation should look. First it says for a long time that the mother transfers to the daughter, until suddenly the man who is supposed to pay is mentioned, who then is, at some point, mercifully allowed into the land register.

Unfortunately, many numbers are missing. Because if you are unlucky, the daughter simply does not have the income to implement the project alone without the man. Hence all these complicated variants.

if they give the €400k to the mother, then the daughter inherits it in the event of death, but if the mother blows it in 10 years, then there is nothing to inherit, right?

now the other thing:
they choose the construct, €1,400 until the €400k is paid off, how is it then regulated if the mother dies after 5 years?
by then they have paid off €84k (daughter €42k, partner €42k)

thus the partner then owns 50% of a property, let's keep the value at €400k, so his share €200k, although he only paid €42k for it

that’s the point!

therefore the option to pay €1,400 until the €400k is paid off is nonsense, because it is not properly regulated. In this variant, the mother and daughter are massively disadvantaged compared to the "daughter's partner" both because they save the interest (which can be seen as a gift to the daughter) and in case of the mother’s premature death before the €400k is received.

that is not an insignificant aspect.

therefore the variant:
they pay €400k to the mother (of which €200k is paid, €200k is given to the daughter as a loan)
€200k is given to the daughter by the mother as an interest-free loan, she pays it monthly with €700 (exactly what they plan)
the partner must pay €200k out of his own pocket, if he doesn't have it, he must finance it! That’s just the way it is...
the mother can receive interest for the €200k cash she gets (so she does not make losses)
if she dies and there is still some of the €200k left, the daughter inherits it

very simple and fair for all parties
and the "partner" pays the usual interest, which is also fair.
Of course, it could also be arranged that he takes the €200k as a loan from the mother at 2.50% (still saving something compared to the market interest) and pays it off with €700 per month. Such things must be regulated.

because if the mother dies after 5 years, then he must continue to pay his loan share to the daughter until his 50% is paid off. Very simple, no one is disadvantaged by that

now the other thing: they choose the construct, 1400€, until the 400k is paid off, how is it regulated if the mother dies after 5 years? by then they have paid off 84k (daughter 42k, partner 42k) thus the partner then owns 50% of a property, let's keep the value at 400k, so his share is 200k, although he only paid 42k for it it’s only about that!

That’s all clear. But if the mother granted a loan of 400,000€ that is serviced every month with 1400€, then the loan continues to exist after the mother’s death. The loan is inherited! With the death of the mother, nothing changes for the man, he still has debt. Now just with his partner, instead of with the mother.

that’s why the option to pay 1400€ until the 400k is paid off is nonsense, as it’s not properly regulated.

No, the loan agreement is the sensible regulation that remains valid after death. quite simple Addendum: I agree with you to the extent that the contracts and loans should be made independent of the man and the woman, and not signed as joint debtors.

thus the partner then owns 50% of a property, let’s keep the value at 400k, so his share is 200k, although he only paid 42k for it

then the partner still has a loan obligation of 158k towards the mother's community of heirs. Just because the mother is dead does not mean the loan disappears for him.
Possibly it disappears for the daughter’s share if she is the sole heir (confusion).

Sale to both as co-owners at 50% each should be unproblematic (subject to 50% property transfer tax)
Set the purchase price as a loan. I would always include an interest rate here (around 2%), then there would be no disadvantage for the mother either because she wouldn’t get a higher risk-free interest rate on the capital market.
Alternatively, possibly tax-optimized and higher interest on the V+V commercial unit share.