I had to sell a dozen of my .de domains to pay for debts. But I had it easy that time: I already knew someone interested in the domains and had my admin-c handle all the technical stuff. I just signed the contract, waited for the money and then gave my admin-c the "OK, do your domain-thing".
In Germany it is important to have a written and signed contract, stating exactly what was sold to whom and for which price. And I would also include excplicitely things that are NOT sold in this contract: only the .de-domain but not the .net- or .com domain.
It is important to be clear, and the contract does not need this fancy lawyer speak to be valid. It needs to be understood by both parties. Use simple sentences to make things clear. Legalese is for lawyers, German is for Germans.
That said: I think the contractual part is much more difficult than the technical part of transferring the domain. Also the contract is the most important part, in case something goes wrong and some weird dude (GvG are you listening?) registers the domain during transfer.
I wonder if denic.de has instructions on the technical part...