To transfer available credits from an existing vendor to another, transfer the amount (from existing vendor) as credit to a Clearing account and debit the amount from the Clearing account to the other vendor.
Step 1: Create a clearing account
Step 2: Create a journal entry to move the credit from existing vendor to wash or clearing account
Step 3: Apply the credit to the General Journal Entry created as an unpaid bill
Step 4: Create a second General Journal Entry to move the credit from Clearing account to the vendor that will receive the credit.
The credit should now appear on the vendor's account and available to be applied to outstanding bills in the Pay Bills window.