From the Sales Screen, you can click on the "Revisit" button which is located on the right hand side of the screen.
Once you have click on the "Revisit Screen" the system will show you a list of parked transactions, locate the transaction that you would like to re-open.
Once you have located the transaction that you would like to re-open, click on the icon that looks like an eye, to preview it and verify that the right transaction has been selected.
Once the Preview screen opens, your products with the quantities and prices they were parked with, will show. If you have open the right transaction, you can click on the "Continue Sale" button to load the parked transaction. Otherwise click on the Close button until you have selected the right transaction.
Once the transaction has finish loading, you can continue with the invoice, just like when you parked.
NOTE: The transaction will remain under the Parked Transaction list, until the transaction has been finalized.