Buy property in advance with family advance payment

And variable loans are usually somewhat more expensive (higher interest rate), if I see that correctly, right?

Definitely talk to your house bank about it and also present the future financing. I had several banks compared with variable financing through a loan broker, and the interest rates were significantly higher. I was told that banks don't like variable financing. In the end, my house bank was 0.75% below the other offers and is naturally looking forward to follow-up business.

There will be no follow-up financing, as it is to be fully repaid after the move through the proceeds from the sale of the current inventory.