Double costs for the bathroom / who has to pay?

To clarify terms: you are likely to have a general contractor. A property developer always sells a house in combination with land

and the property developer can also hand over the house unfinished, so that some trades are subsequently coordinated by the builder himself. Of course, in the situation described by the OP, it could also be a property developer.

Learn: never pay everything in advance!

Apart from the fact that this sounds quite patronizing – it is unclear whether the 6000€ was a deposit or full payment.

Then it will probably come down to us having to sue the bankrupt craftsman, hoping to recover something.

If the "bankrupt" means insolvency, you don’t need to sue. Get a judgment and see what comes of it. If he has not filed for insolvency, talk to him and demand a refund with the friendly notice that otherwise you would file insolvency for him. You don’t need a lawyer. That will only cost you extra.

I thank you all in advance for your quick and detailed responses. The transfer went directly to the craftsman. It only concerned the special requests but had to be coordinated with the developer. This is because (Beispielszahlen vertraglich 5000€ für das gesamte Bad bereits im Kaufpreis inkludiert ist. If we now generate a total cost of 15000€ for the entire bathroom, these 5000€ would have to be paid by the developer, since according to the purchase contract he spends that amount and also receives the entire purchase price from us. The topic is really hard to put into words so that people who were not involved can understand it.

The topic is really hard to put into words so that people who were not involved can understand it.

Well, therefore the contract would be quite helpful.

Basically, everything regarding special requests that was discussed and paid directly with the craftsman is your thing. (This is probably also what the contract will say).

What would still be the question are the "basic costs." Are we still talking about the bathroom fixtures (sink, fittings, etc.) or about the rough installation?

How is the assumption of these costs formulated in the contract? Do you commission the trade and the developer takes over 5k or how?

I already thank you for your quick and numerous answers. The transfer went directly to the craftsman. It only concerned the special requests, but had to be coordinated with the developer.

If it is only special requests that were agreed upon between you and the trade, the matter is quite simple. This is a normal procedure, your construction company has nothing to do with it, and unfortunately it is your "problem".

If it's just special requests that have been agreed upon between you and the trade, the matter is quite simple. It's a normal procedure, your construction company has nothing to do with it, and unfortunately, it's your "problem".

Yes, I had already thought so. Does it also not matter that we couldn't choose this craftsman ourselves and were assigned him?

Yes, I already suspected that. So it doesn’t matter that we couldn’t choose this craftsman ourselves and were assigned him?

Yes, that doesn’t matter. You spoke with the craftsmen, agreed on the special requests, received an invoice, and paid it.