The main rules on how to keep a good terms with your customer: Set a clear boundaries. It's important to keep a clear understanding of what you promised and let the customer know when to stop to ask for something extra.

Always negotiate. If the customer wants something beyond what you agreed on, tell him it's possible, but you have to give up something else. The other option is to turn it into an upsell and get some extra money. Never give up something for nothing. Give clients more options so they can keep them in mind. There might be a case where you can say: hey, we'll give you two extra developers. It'll cost you twice as much, but you'll get your app twice as fast. It won't work for some, but others will use it or refer to it later on. Be firm but fair. It is important to communicate your limitations to your clients with firmness, but you should also be fair and respectful. You'll have a better working relationship and less conflict. Don't be scared to lose customers. Whenever there is a problem with a customer, you can always say that it's over and stop the development process. The best option is to do a full refund and assist him in working with another developer,