Remove app from phone, but save a file or let server know app is uninstalled.

Hello,

Is there a way to let the server know if the app is uninstalled?

And is it possible to leave some files at the phone when uninstalling the app?

I want to use a credit system on my app.

Credits are linked to a user by a unique key that is stored in the sqlite db on the phone at first run.(saved by a request from server) and at the server. (i use a unique key because I don't want to store the users telephone number at the server due to privacy reasons.)

When a user removes it i want to know about it so i can remove the unique key from the server or save it(for lets say a year) when a user has bought credits.

Any ideas about a credit system are welcome. How to handle stuff when user buys credits, removes the app and installs it again..

Greets, J.