Unable to create a price correction credit memo

Hi,

We're trying to create a price correction credit memo for an invoice matched to a PO. The PO shipment amount is 174.20 and the invoice line amount is 304.80 (Tax of 57.91). When trying to correct the unit price of the invoice (price correction unit price of 13.06), I'm getting the below error

APP-SQLAP-97747: You cannot reduce the amount billed for the purchase order shipment or corrected invoice to less than zero. Please reenter the correction unit price

The corrected invoice is in 'Needs Revalidation' status and the PO is closed

Any ideas to how to proceed or what to check?

Thanks,

Shreyas