When you enter the first goods receipt, $880 is posted to the GR/IR clearing account. When you enter the first invoice, the corresponding share is cleared for 60 pieces. When you enter the second invoice, $220 for 20 pieces is still open on the GR/IR clearing account. This is cleared and the remaining 20 pieces are posted at the invoice price, that is, 20 x 13.70 = $274. The next goods receipt is also valuated with the invoice price of $13.70/piece.

When you post the credit memo, there is still an excess invoice quantity of 10 pieces. This is multiplied by the difference between the clearing value (660 + 494 = 1154) and the goods receipt value (880 + 137 = 1017) and divided by the difference between the invoice quantity (100) and the goods receipt quantity (90). The remaining credit memo quantity of 50 pieces is multiplied by the goods receipt value and then divided by the goods receipt quantity. The result is the amount that is posted to the GR/IR clearing account when you post the credit memo.

After you post the credit memo, there is a balance of $565 on the GR/IR clearing account. This means that the system still expects an invoice for a goods receipt of 50 pieces. When the invoice is posted, the balance is then cleared.