Sucinto on Nostr: If someone is a writer(or any kind of home office job), and charges 10k sats an ...

If someone is a writer(or any kind of home office job), and charges 10k sats an article, and advance payment of 5k sats, 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 10k sats?