Home financing with a condominium unit still for sale.

since the buyer of your house then acts as the landlord, he also takes on all the duties of a landlord and in case of doubt even has to sue you out first (if you don't move out by the deadline, for example), he will certainly not settle for 120€ a month. he also has financing for your house and does not want to start at a loss.

you can discuss many things with the buyer, such as a later handover and a lower sale price in exchange. it is best to simply talk to the buyer. if there is no agreement, then you have to arrange bridge financing.

You can contractually agree on the handover date of the apartment, can't you? Until the handover, we would continue to pay the service charges and utilities and instead of paying rent, deduct that amount from the purchase price. Is that possible?

Yes.

The money for the sold condominium, however, could only flow about 2 months after the start of construction of the house. Accordingly, the equity would be contributed later. Is that a problem for the banks?

For a good bank, no (big) problem. In case of doubt and if interim financing is necessary, the bank can have the claim for payment of the purchase price assigned.

A bank that only does standard 0815 business can lead to (resolvable), but annoying complications.

No problem at all. In order for the apartment to be considered equity, you also don't need to present a buyer. The market value of the apartment alone is decisive. One idea would actually be to take the amount (or slightly less, depending on how much you really need directly) on the existing apartment if there is still a land charge. This would make the "big" loan cheaper as already said.