A void is canceling a transaction that you just processed, but has not been settled (this does not cost you anything to do this). 

A return is when you have already settled a transaction and want to refund the customer there money back (your bank will more than likely charge you a charge-back fee). Note, you would need to ask your bank what that FEE cost is. 

Select Unsettled Transactions from Home Screen	Highlight the Transaction you would like to Void - Tap Menu then Void	Confirm Void amount