Android mobile app development for invoicing

We have a mobile app with various functionalities that integrates with our core service.
We want to add a capacity for invoicing on the app we already started building. We have a design for.
We are looking for a developer to finish it.

There are about 6 screens to be developed.
- Enter invoice
- Add invoice line
- New/Edit contact person
- Chat with person and call person
- New/Update catalog entry