How to protect an android application in freelance development?

Question

Android

Good afternoon.
There are freelance exchanges, and there everything is more or less clear with protection against fraud on the part of the customer (although, perhaps, it does not work effectively enough).
Tell me, please, how can you protect your application from theft or deception on the part of the customer, if the correspondence / work on the project is carried out outside the exchanges?
UPDATE 1: a question, in particular, about a situation where the client is not interested in the source code, but in the product itself as an application.
PS: I myself am a wannabe Junior Android Developer, I received the first order, because. in his hometown tight with vacancies. I really hope for your help, dear experts.

Answer 1

The easiest way is to check the date on your smartphone every time you start it. If the date is more than the agreed payment deadline, the application refuses to work.
You can limit the functionality - implement everything ordered, but cut out some of the functions during the demonstration.
After payment, you give away a fully functional application.