How to refresh user balance after paypal payment

How to refresh user balance after paypal payment

  
I have a "buynow" paypal button. I already set up the IPN Listener, set up and tested everything (sandbox testing) and are all fine.
But there is one very important thing missing. My IPN Listener needs to receive the username (or user id...) of the user of my app who made the payment (the username in my app), in order to refresh the balance of that particular user in my database.

I think that somehow when the button is pressed I must send to paypal the correct username or user id, and paypal IPN server sends it to my IPN LIstener, in the notification message.

How can I do that?