JAVA Gaming,true tone server question

I am developing an application where u can download true tones java games etc.

First user sends an sms with the id of the content and then as a reply a wap push msg is sent back to you.

The wap push msg has the url to download the file.

1)Is it possible to understand if the file has been downloaded or not?

2)I can charge the user by a proxy system but i have the question in my mind
if i charge the user before he may not downloaded yet and may have problems.
if i wait till the download (if i can understand that) the file may be downloaded but the user has insufficent credits..
what can be done?

THANX

"THERE IS NO PATCH FOR HUMAN STUPIDTY..."