How to get an Open Voucher Balance for a prior date: 

In Purchasing choose Reports > Voucher Detail Report

·         Enter the Fiscal Year you are looking to run to

·         Remove the usernames in the 'Entered By' fields

·         Under Include – Choose 'Open and Held' – Enter the desired As of Date

The Screen shot below will give a report of open vouchers as of 05/31/2015