I am not sure how that would work. I've had one return with this system and it gave me the option of refunding with or without the original shipping cost but I don't think there were any other options.
I do know that the return will close on its own after approx 30 days even if you approve the return and the buyer doesn't return.
Perhaps you could have the buyer send the items back without inputting any tracking numbers so that ebay will not know when/if the item is returned. When they are returned you can do the partial refund through paypal (partial doesn't give a defect).
Basically you would be bypassing the return system but you might want to ask your question on the .com powerseller board as some of the posters there seem more familiar with the return system. I'm curious what would happen if you don't approve or acknowledge the return at all.