Steps to re-create:
~ Create an invoice
~ Click "Allow Partial Payment"
~ Enter minimum payment
Let's say that the invoice amount is $1000 and the minimum partial payment is $500.
When you go view the invoice, you will see the radio button option to make a $500 partial payment. If you select that radio button, the "Process Payment" button still displays the button text of "Process Payment of $1000". -- I would have expected that some function would update the button to read "Process Payment of $500" instead.
So using this same example, if you click on the "Process Payment of $1000" button, you are redirected to the PayPal website as expected, but the amount to be paid is for the FULL invoice amount, NOT the partial payment amount. Furthermore there is NO OPTION on the PayPal page to make a partial payment at all.
Bottomline, this feature does NOT work!