Purchase order approval attribute on original order amount

Summary:

I need to create an approval rule for a change order based on the original PO amount.

Content (required):

Hi everyone,

I need to create an approval rule for a change order that uses the original document amount and the new document amount. I can use the Approval amount for the current document amount. However, I can't seem to find an attribute that can be used for the original document amount (PO amount before the current change order). The information must be stored somewhere, because I can use the "Change in document total amount", which is the difference between the new document amount and the original document amount.