If you are having trouble vouchering a Purchase Order because there are Returns logged against it, then you probably left out an important step: Creating a Credit Memo for the Return. To do this, go to the "Uninvoiced Receipts/Returns" report screen and right-click on the Return record. In the right-click menu is the option to create a Credit Memo for the Return. Once the Credit Memo has been created, you should be able to continue processing your Voucher. Later, you can apply the Credit Memo to an open Voucher.