Once the G/L is out of 'History' mode, and a G/L account is linked to inventory, transactions to that account can only be made through the Inventory module (meaning Payable or Receivable invoices, Inventory assembly transactions, bill of materials, or Adjustments).  

Before taking the G/L out of 'History' mode, it's possible to just type in the beginning balance.

To make adjustments to that account, you would have to remove all the inventory items that are linked to that account, which is probably not possible if the items have been used.

Depending on the Edition of Sage 50 that you have, you may be able to create multiple inventory accounts, so that you could have one to record such things as financial adjustments and work-in-progress.