Real estate financing, help, Riester available, Bauriester not available

Good day, I am now 47 years old. Since I have gone through a divorce, I had to start over at <0.
However, I now have the opportunity to buy a property, but I just don’t know the best way to do it....

About my situation: I have a monthly net income of 2500 Euro and I am not obligated to pay alimony. Since I currently live very simply and cheaply, I have about 1000 Euro left at the end of the month.

My finances are as follows:
Savings account: 18,000 Euro
Riester fund: 11,000 Euro
I contribute the maximum subsidized amount of 2400 Euro annually to the Riester fund.

Now I have the opportunity to buy a two-family house from a friend for 130,000 Euro. The house is to be fully rented out, which generates a monthly rental income of 900 Euro. He has to sell the house because his still wife wants CASH after the divorce.

First, a question that can probably be answered with a clear yes or no. Would it be legal to have the purchase price appraised at 110,000 Euro to save fees and then give the seller 20,000 Euro in cash???

I intend to pay off the property within 10 - 12 years and at the latest then partially use the property myself.

Now my actual question.
I would like to invest the Riester contract entirely into the house without losing allowances and taxes. I just wonder how this can be realized, because I do not want my Riester contract to be converted into an annuity. In the next 10 years, at least another 24,000 Euro would go into the contract.

How would you finance such a property? And which BSK would be recommended in this case???

You can withdraw the money including allowances for home ownership from the contract.

You probably won't get tips on tax evasion here.

Wohnriester is intended for owner-occupied properties. If these are rented out or sold, it gets tricky. In the worst case, the received bonuses and tax benefits must be refunded. I would inform myself very carefully whether this makes sense in this case, especially if renting out is intended from the outset.

When money is involved, friendship ends, so you will hardly get any tips on tax evasion here ;).

and the 20k CASH is supposed to go to the buddy then? Everyone has to pay taxes. I do not support this thread here.