When you reduce invoices, the system creates an invoice and a credit memo simultaneously. When you accept the quantity variance, $1000 is posted to the GR/IR clearing account in the invoice (the system expects a further goods receipt of 30 pieces). The difference of $200 to the invoiced amount is posted to a clearing account for vendor invoice reduction.

The posting to the clearing account for vendor invoice reduction is cleared in the credit memo that is created. The offsetting entry is posted to the vendor account.

The tax posting in the invoice is based on the item amount entered. The credit memo corrects the tax posting.

Together, the invoice and the credit memo create the liability that results on the basis of the changed proposed data.