Stupid situation, but the KfW is a normal bank, I wouldn’t expect goodwill there.
Especially since the argument is strange, I don’t get a discount from the general contractor just because interest rates are so high now, nor does my bank generously give me 10% off the amount for the follow-up financing.
I had to pay tuition fees back then, and when I was done they were abolished. That’s just how life sometimes is.
That the KfW adjusts the interest rates on such loans according to the market is stated somewhere in the contract or is known. In return, the interest rate is fairly favorable and you don’t pay any prepayment penalty.
No offense, but you can’t always just take advantages and when the tide turns everyone is mean and you hope someone will give you something for free or waive something.