Sucinto on Nostr: If someone is a writer(or any kind of home office job), and charges 0.5 XMR an ...

If someone is a writer(or any kind of home office job), and charges 0.5 XMR an article, and advance payment of 0.25XMR, how would the client assure that he would not escape without doing the service, and if he does, how would the writer assures that he would be payed all 0.5 XMR?(without a third part)