The UOM on the item on the PO is in gallons but since the supplier only uses pounds, the user conver

The UOM on the item on the PO is in gallons but since the supplier only uses pounds, the user converted and entered the wrong quantity. 

The item has already been received and the invoice has been paid ($14k++). 

The user attempted to correct the PO by adding another line with the correct conversion (lbs to gal) hoping he could delete the original incorrect line. It will not let him do so.

Now there are two lines and the PO amount is doubled to $29k.

What is the best approach to making sure the PO information lines up with the invoice?  Does he have to re-do both the PO and invoice?  Can we just leave it as-is?