ᐅ Kitchen deposit, fixed pricing – price increase after expiration?

Hello everyone,
we purchased a kitchen with a 2-year fixed price guarantee. Unfortunately, we were only able to place the final order one year later.

Now we are being asked to pay nearly 3.5% more because the price guarantee has expired. This is not a problem in itself, but we also made the down payment 3 years ago, and the lost interest represents a similar amount.

My idea is that we pay the difference between the lost interest and the price increase, since the kitchen company was able to use the money for 3 years. The kitchen builder’s argument is: "I couldn’t use the money because I had to cover my ongoing costs." It is worth mentioning that nothing was ordered in advance by the kitchen builder.

The contract does not specify what happens after the price guarantee expires. At the moment, I simply feel doubly disadvantaged because I couldn’t invest my money for 3 years and now have to bear the price increase alone. We’re talking about nearly 3,000 € in total.

What do you think about this situation?

I am very frustrated that we signed the contract so early and also made the down payment, but I cannot change that anymore. My first impulse was to just let them keep the kitchen, but I’m afraid the down payment would be lost then, which would mean a loss more than four times as high.