New broker law when buying an existing property?

means about €5000 more cost, which we lack for the necessary renovation work.

If you want to have the house, then I recommend you grit your teeth and pay the extra €5000. If necessary, you can even get it as a consumer loan everywhere.

Or you try to save the entire broker fee by revoking! But first you say:

We ourselves did not conclude a contract with the broker.

and then

What we still have is a, I would call it informal letter (A4 page) entitled “Declaration of intent to purchase” in which, under the item broker commission, the former 5% is manually crossed out and replaced by 2.5%. Unfortunately, we already signed it on 13.12.

Maybe your memory plays you another trick and there actually is the revocation instruction.

Sounds like saved realtor commission to me.
As soon as the contract is completed at the notary, the invoice from the realtor will come.
Then you send the lady the cancellation and that's it.
€5000 more for the renovation than planned.