Pay off loan contract early?

Hello dear users,

I'm 23 years old and want to get married in 1-2 years. My parents want to emigrate from Germany in 2-3 years. To secure our future (and our siblings), they've started a construction project that will be completed by the end of the year.

This will result in four spacious apartments. One of them belongs to my sister and brother-in-law, two will be sold to pay off the debt, and they want to leave me a penthouse apartment with a loan balance of €380,000 at an interest rate of 1.3%. The financing costs are €1,200-1,300.

My idea or vision would be to live in the penthouse apartment for two years and then sell it to make more profit. Once built, the property would be worth around 900,000 euros, depending on the market.

Now the question is how high the compensation for the bank would be if I decided to sell the property after two years and thus pay off the remaining debt early, which would amount to 350,000 euros, considering that I have already paid off a sum of 30,000 euros in the two years.

With the profit from the sale of the property, I would take out a new loan to build two terraced houses. Since my father-in-law owns a construction company, I would save a huge amount of money and minimize my debt as much as possible.

When the construction is finished, I would sell one of the houses to pay off my debts, so that I could then live in the one house debt-free or almost debt-free.

What would be your advice regarding the performances?
and how much would the compensation cost if I pay off the debt early by selling the penthouse apartment.

with kind regards