How would you give away free bitcoins by letting the user just scan a qr code?
You would have to do this in a couple of steps, the first of which doesn't involve Bitcoin at all.


User scans QR code using an app you provide.
Your provided app sends a request to your server. This request contains something that identifies the user (say, the user's Bitcoin address).
Your server sends Bitcoin to the user's address.


Having said that, if you already have the user's Bitcoin address, then you should consider skipping the QR code entirely and proceed directly to step 3.