Kitchen seller wants the remaining payment in cash, does not want a transfer

for 6 months it has said "Page is under construction"

Did the page ever exist? If yes, google wayback machine; they make a backup of the internet every few months.

It should also be on the invoice/contract.

If cash payment is agreed upon in the contract, I would strongly adhere to that. Queasy feeling or not.
It’s best to have witnesses present and have the payment acknowledged in front of them as well.
Then you’re safe there for the time being.

Warranty claims are a different matter. If he is insolvent, you might end up empty-handed regardless of the payment method.
Otherwise, his accounts and garnishment issues are not your concern.

In the event that insolvency proceedings are underway, the insolvency administrator will be able to insist on renewed payment. Rightly so! Then you would have to pay your 10,000 twice... so make sure you find the Handelsregister-B (should be on the business paper of the contract) and if necessary, ask the local court.

In the event that insolvency proceedings are underway, the insolvency administrator will be able to insist on a repeated payment. Rightly so! Then you would have to pay your 10,000 twice... so make sure you find the Handelsregister-B (should be on the contract’s business paper) and if necessary, ask the local court.

I don’t understand, what do you mean by that? If she has a receipt that she paid the amount in cash as contractually agreed, why is an insolvency administrator allowed to insist on a repeated payment? Assuming the case that no insolvency proceedings existed at the time of payment? That can be checked, and the OP says that there is currently no insolvency proceeding.

I assume that once the insolvency is opened, the former boss is no longer allowed to collect any money since he has no more rights.