On the PayPal product family, take a look at Adaptive Payments. One of the features is the ability to do what's called Implicit Payments. Essentially this feature givves you the ability to programmatically send money out of your own account, to pay somebody else.
You simply need to write the script to determine the receiver's email address, the amount you want to send them, and make the API call with that information.
Check out page 25:
https://cms.paypal.com/cms_content/US/en_US/files/developer/PP_AdaptivePayments.pdf
Although it's probably easier to write your own API call (It's basically and HTTP POST request, with some additional headers), you could probably tweak the ActiveMerchant library to do it for you.