House purchase - split the price 50/50

Under 50k you will hardly get the good mortgage rates. For many, the minimum amount

That's true. Because you still have to pay the notary and the land registry ordering and registration. You just have to calculate how much more you pay. No idea where 5 years interest rate lock at 50% loan-to-value stands. Estimated at 0.7%. In contrast, there is a consumer loan with 3% or more! Does that make sense? Variable possibly at 1.2%.

If you have a decent saving performance, I would always prefer a variable loan. We financed our land variable and after one year reduced it to 55k€ through complete special repayment and paid 300€ interest. Stretch that over 4 years if you like. No consumer loan can compete with that.

If you have a decent savings performance, I would always prefer a variable loan. We financed our plot variably and paid 300€ interest on 55k€ after one year through complete prepayment. Stretch it over 4 years if you want. No consumer loan comes close to that.

I have given some thought and read up on the variable loan. The Euribor is used as the interest basis there, if I understand correctly. So are variable loans then not different from bank to bank at all, because they all have to orient themselves to the Euribor, or how is the Euribor to be understood? Because according to my research, it is currently at -0.31% for 3-month money. Since we will borrow about 40k, a building loan is out of the question, and the variable loan can then be repaid continuously.

So variable loans are not different from bank to bank at all because they all have to orient themselves to the Euribor, or how is the Euribor to be understood?

It depends on what a bank understands by "variable." There are not many banks that offer consumers Euribor + margin + liquidity costs. Normally, there is also a floor on the Euribor, i.e., the negative base rate is not passed on. Apart from that, the 3-month Euribor will also be abolished at some point. Then the Eonia could become the basis (already possible now). Or the bank offers based on the ECB key interest rate. Or or or.... In the end, there will be a price on your variable loan that has a base of 0 plus the margin charged by the bank. Given your loan amount and the short term, as a bank I would maybe add 2-3%.

and then I agree on a monthly installment, and depending on the current interest rate, the repayment portion of the installment is higher or lower? Is it common to be allowed to make special repayments monthly, for example, if there is still some money left at the end of the month after savings, deductions, etc.?

and then I agree on a monthly installment

You agree on an installment repayment, i.e. the repayment amount is the same every month/quarter (or whatever is agreed upon); however, the interest amount changes. It is completely different from a classic annuity loan.

Is it common to be allowed to make special repayments monthly?

That is the purpose of the variable loan. However, if you conclude it based on the 3-month Euribor, you can only repay every quarter.

Even companies do not get negative interest. Only common within banks, e.g. repo transactions