In this specific situation, the price I wanted for the project was too much for the buyer and he suggested to pay in partial payments. I don't know this person and don't know how trustworthy he is.
How could I make that work? How can I protect myself? Selling a domain is easy using an escrow service, but how to send code and other assets safely and so on?
Ensure that assets belong to you before the full payment, but also ensure that they get everything after full payment. Both of you should sit with the lawyer and discuss it.