Referral Link and Pay User for Referral

[skylab]

I have a paying subscription based board with 3-month, 6-month, and 1-year setup. I'd like to setup a way to pay my users who refer other people based on how many months the user purchases. When the user clicks on a referral link, I'd need it saved in case the user registering decides to surf around the site then register after. Using paypal would be best.

Example: My 3-month costs $30. I give you my referral link and you buy $30 as well. I get paid $15 for referring you.

How could I make this happen? I tried searching a few mods but haven't found something so exact.