Temporary VAT reduction by 3% until 31.12.2020

Correct, there is no law that requires the entrepreneur to pass on the VAT reduction. And to my knowledge, it is not part of the draft law either (without having read it myself). However, since it is a pass-through item for the entrepreneur, various politicians have appealed to entrepreneurs to pass these amounts on to the consumer. That the process involves an incredible effort for companies, which will not lead to any additional sales for many products, is often overlooked by politicians. On the other hand, some companies that do not pass it on are certainly going to be exposed to a wave of outrage. So what is certainly intended to be nice for the consumer probably just costs the economy overall in terms of resources. I consider this measure a brilliant failure. So a grade of 1 for the idea and a 5 for the execution.

I fully agree with that.

In our construction contract it says:

"Any possible increase in value-added tax will be charged additionally"

any possible decrease in value-added tax is not mentioned

I refer you to the neighboring thread in the post by face26 #79 point 4 and the following posts
https://www.hausbau-forum.de/threads/mwst-senkung-waehrend-des-hausbaus.35217/page-14#post-411254

For long-term contracts, it depends on when the contract was concluded whether a compensation according to §29 USTG must take place or whether civil law applies. However, this only applies if nothing is written about it in the contract. In your contract, the VAT increase is regulated, but not a reduction.

Although your contract regulates the VAT increase, it does not cover a reduction.

In the case of such an asymmetric clause in a consumer contract, I would first credibly threaten legal action. Then the entrepreneur will quickly pay the 3%... The likelihood of you winning is quite high. Or better, simply transfer the amount with 16% VAT and reject the rest as an unjustified claim. Then see what he does.

In the case of such an asymmetrical clause in a consumer contract, I would first very credibly threaten legal action.

...or maybe one should first ask how the contractor intends to handle it?