Close Partial Paid Voucher and Restore Remaining Encumbrance

Client has a partially paid voucher and wants to restore the remaining encumbrance back to the PO. In a test environment they closed the voucher and restored the encumbrance but it restored the total encumbrance and not just the remaining amount after the payment. I can't find clear documentation saying if upon closing it's all or nothing or if it should have restored only the remaining amount. Does anyone know the answer?