PO Return - wrong UOM leads to high total value

Dear Gurus,

I have an issue concerning PO-Return (ME21N). The End user created a mistake in the PO's UOM.

instead of 1 can (15KG) per 1 can it was 1 can per 15 car. the total price of that material should have been 280USD but it became 280 * 15 = 4200USD.

Goods Receipt (movt.161) was done

invoice verification (credit memo) completed.

this is a 3 months old document so i cannot reverse MIRO.

Is there a way to decrease the PO total without reversal in MIRO? i tried subsequent debit/credit but there's a message "System cannot calculate cash discount base amount; enter".

Any recommendation will be highly appreciated.

Thank you.

Regards,

Ber