Chat/Payment App (Backend and App development)

We are looking for a programmer to develop an Application (JUST FOR ANDROID) and the associated web based backend to manage the App. The App will be used as a contact tool for clients who will make queries and request reports / documents. Something similar to a point-to-multipoint whatsapp interface. We need the basic features of modern chats, like be capable to send not just text but files or pictures. The capacity of a client to send files and pictures must be able to be restricted if necesary (thats beacause some clients abuses of that feature). The App must have a loading/splash screen, then two tabs will be show: Main tab: it will have information. A hierarchical list that can guide user to specific information. Its just "additional content". This list and the information to that it takes must be handled from the back end (added, erased, modified, etc). Secondary tab: this query/request tab will be the heart of the app and will consist in the mentioned chat with the mentioned features. But with something else that is not typical: "hidden message/files until payment is done". The idea is simple, some messages or files that the server sent to the client must stay impossible to read until the client pay for that. Then, every thing that server sends to client must be able to be set to unreadable/undownloadable. The last feature is payment. Client must be able to see the unreadable/undownloadable and the server's stablished price to make it available to read/download. At the same time be able to pay for that from the App. For example clicking the message/file and geting a payment method (dont know wich is the best way to do it in mobile apps, I mean friendly an easy). All chat conversations and info for first tab should be able to be handled from the backend in an easy, fast and individualized way. Backend must be host in a place like AWS, and must be complete functional (database development, and all whats required for the backend to do the job).