KfW BEG funding stopped 261, 262, 263, 264, 461, 463, 464

I would talk to the general contractor, my goal would be to mutually remove the clause and leave the contract unaffected and proceed with construction.

Well then, you unfortunately made it too easy for yourself. As for me, I had it in the back of my mind – what I didn’t realize, however, is that we have the exact numbers stated in the condition...

I would talk to the general contractor,
my goal would be to have the clause removed by mutual agreement and to leave the contract unaffected and build.
Or
Anyone relying on the subsidy may possibly have a cheaper exit to limit the damage

I don't think that would be legally secure. You would basically be modifying a contract that has already expired/invalid/non-existent. I would definitely consult a lawyer about that beforehand ;)

A waiver declaration is currently being drawn up...

The exact wording of a suspensive or resolutive condition is at the discretion of the contracting parties. The following sample wording of a suspensive condition is, however, recognized by the two implementers BAFA and KfW:

"The obligations for (delivery) services provided for in this contract serve the implementation of [eines Sanierungsvorhabens / eines Neubauvorhabens], for which one of the contracting parties has applied for funding under the program 'Bundesförderung für effiziente Gebäude' (BEG) of the BMWK at BAFA or KfW [applied for [has/will apply within […] days after contract conclusion]].

Suspensive condition:
This [purchase contract enters into force / contract enters into force with regard to the delivery and service obligations for implementation] only and to the extent that [BAFA / KfW] approves the application [only for purchase contracts: for funding [designation of individual measure / a renovation project / a new construction project]] and has granted the funding with a commitment to the applying contracting party (suspensive condition). The applying contracting party will immediately inform the other contracting party about the occurrence and scope of the occurrence of the condition.

Resolutive condition:
This [purchase contract expires / contract expires with regard to the delivery and service obligations for implementation] as soon and to the extent that [BAFA / KfW] does not approve the application for funding [designation of individual measure / a renovation project / a new construction project] but rejects it and denies the funding with a rejection notice to the applying contracting party (resolutive condition). The applying contracting party will immediately inform the other contracting party about the occurrence and scope of the occurrence of the condition."