approval template for line item price variation

Any recommendation on how to managed price variations at a line item level with using approval templates? The approval templates currently only allow for TOTAL document differences.

A senario as follows

1. Purchase order with line item price created

2. GRNI created (and unit price changed)

3. AP invoice created - AP approval template kicks in advising that there is a price discrepancy at a line item level between the Base and target documents.