Is a deviation in market value possible when selling real estate within the family?

It only has to be in the case of the legitimate interest in an alternative economic use quoted above. What exactly constitutes such a legitimate interest is not clearly defined. For example, the sale is always meant and only that is apparently accepted by banks and other institutional lenders without dispute. I wanted, for example, to rent out my condominium and refinance it and restructure the loan. However, ING did not accept this either. Fortunately, the KfW share was sufficient for the new bank for the entire project...

But is the procedure of the insurance even legal? If that were the case, a land charge would never be deleted. Logically, the bank only has a right to the land charge as long as the loan is not repaid. If I now offer the bank to repay the loan plus prepayment penalty, it must also accept this and accordingly approve the deletion.

No. It does not have to. There is no claim to this - only in case of sale. Since my training, I have practically never come across any other legitimate interest - so since the early 90s.

Logically, the bank only has a right to the mortgage as long as the loan has not been repaid. If I now offer the bank to repay the loan plus prepayment penalty, they have to accept this and accordingly approve the deletion.

You have signed a contract for X years and the bank wants you to stick to it. In this case, the OP is the one who wants to change the contract. That the bank pursues other goals besides the money for this specific loan is annoying but irrelevant.