How do I create a credit memo to decrease the price of an item received by quantity?

We use 3-way matching and have a PO with qty of 1 with a unit price of 45.00. A receipt was entered to receive qty of 1 for the item. The voucher came in for the item with a price of 48.75. We entered the voucher and ran matching - a match exception was found because the price exceeded the PO. We then received a credit memo for the item for -3.75 to reduce the price to the PO amount of 45.00. We are trying to enter the credit memo as an adjustment voucher to relate it to the original voucher. The