Is the amount of the advance payment to the craftsman arbitrarily adjustable?

That's exactly right. If only 4k of construction services have been provided according to the description and 76k are still outstanding, then the big scandal should be postponed a bit longer - as long as the quality has been satisfactory so far. Otherwise, you bring more problems into the house than the "principles" are worth.

My very personal opinion.

If you look at the installments of a property developer, they also call for 60% after the shell construction. It's just pre-financing.

In the contract for work with private individuals according to the Building Code, this (advance payment) is prohibited afaik. With us, the trades also only receive installments because it is regulated that way in the construction contract (and we retain 10% security on each installment, which is also agreed upon).

Just put the architect forward if you don’t feel like clarifying it yourself.

So, I spoke with the plumber. The 10,000€ apparently also includes materials that have been ordered but not yet installed. We have now agreed that I will pay the 10,000€ in 2-3 weeks when all the stuff is installed.

Super..sometimes a simple conversation is worth gold