How to reward members for having their invites accepted in Facebook canvas app? - facebook

Question

I'm using the following Javascript for my users to send requests to "invitable friends" from my Facebook app:
FB.ui({
method: 'apprequests',
message: 'Come play with me etc.',
to: str_ids
},
requestCallback
);
function requestCallback(response){
if(response){
console.log(response);
}
else{
//show error to user
}
}
Where str_ids is a string containing the IDs returned by the invitable friends API. This all works fine.
When I invite a single user, let's say his Facebook ID is 1593894704165626, I get the following response:
request
"1399842883640198"
to
["1593894704165626"]
So here I have a unique request ID and the invited person's Facebook ID. I can now store this data in my database along with the inviter's ID, so I have all the necessary info.
However, now I want to reward the inviter after the invitee accepts the invitation and I'm running into a problem.
If user A and user B both invited user C, then when user C joins the game, how do I know whether to reward A or B?
I see from this question that it's possible to pass another parameter along with the invitation, which should help identify the inviter:
Facebook tracking who Invited whom
The first answer here suggests that you can pass such a custom parameter, but doesn't say how. Facebook's convoluted documentation is driving me insane.
Another thing - even if I send that additional parameter, I think there's a problem - for new users the app redirects to the Facebook page that checks whether they want to allow access to the app, and by the time they're redirected back to the app, the additional parameter will be lost.
Is there any solution to this?