You cannot perform an update on the page using the paypal button as the form action sends you to paypal not back to the page where the data assist code is which is what is required for dataassist to work.
You would need to you use an ajax solution to detect the click on the PayPal button to perform an update, you can find a great primer on using ajax with the jquery library http://www.webicy.com/tech-support/3274-simple-ajax-jquery.html