How to correct a prior Receipt Correction on a PO that did not reverse 'Receive' line

I have a question on correcting a receipt correction.  We have a blanket PO with $4000 ordered on Release 1.  It has 2 distribution lines: $2500 and $1500.  Business has entered the full amount as having received at one point but decided at a later date that they want to reverse the entire amount.  However, when making the correction, they reversed both the Receive and Deliver lines for the $2500 but only reversed Deliver for the $1500 line.  So in the PO, it shows $1500 as having received.  We need to correct this but the PO/release does not show up in the Corrections form.  How do we correct this so that we get the received down to $0?  Thanks.