Look at setting up the tolerances on the Supplier to handle the differences.
This will help to automate the postings within a predefined range.
Hello,
Of course, posting proposal must be in balance to be authorized.
In general, matching is suggesting price from PO, regardless of the matching level (line level matching, as per my understanding, is considering quantity to match from invoice line, header level matching is trying to match full received quantity).
As Shawn suggested above, tolerance setup on supplier will balance the proposal if difference between invoiced value and received value is within tolerance defined, otherwise invoiced conditions (unit price + additional costs) in proposal must be adjusted manually.