Only from BHW financing, how can that be???

Yes, it is important now to get professional advice from someone who knows the legal and contractual pitfalls and possibly finds gaps in the contract.

If it is watertight, your only option is a deal with the provider. The problem could be that the commission has already been paid to the salesperson and is not subject to a withdrawal clause. That means your house provider already had quite high costs, which he will of course try to recover from you in any case.