Scenario with open prepayment & credit note

Please find below the sequence of events happened.

Sequence of events:

1. PO raised = $1000
2. PO paid 100% in advance via prepaid voucher under balance sheet account 1XXXX = $1000
3. Goods delivered for $600 and PO Receipt created for $600
4. Regular voucher created for $600 and settled against the prepaid voucher
5. PO is open for $400 and open prepayment against balance sheet 1XXXX = $400
6. Vendor provided a credit note of $400 to use in future purchase (no refund of money)

We want to achieve the following:

1. Release the open encumbrance of $400 - This can be achieved by closing the PO.
2. Release the outstanding open prepayment of $400
3. Create and post a credit voucher of $400 for future purchase. - This can be achieved by creating a credit voucher.