More complicated sales / construction project

You can have the value of the apartment estimated for free by an agent, possibly list it yourself and wait for the feedback. Then you at least have a rough direction. The amount from the sale proceeds-prepayment penalty can then be bridge financed as a flexible loan, e.g. from the DSL. That's roughly how we will do it too.

I can only recommend you to see an independent financial advisor. They will even calculate the prepayment penalty for free if you provide the data.

Ah the commission pain.... wonderful.