INVOICE AMOUNT field on Payables Invoice workbench

This is on 11i and r12 both.

There is a field called Invoice Amount at Header level on payables workbench. This is a required field where user must enter a value.

Then, there are line level amounts on line tab.  Unlike other modules, Payable maintains no relation between invoice amount at header level and invoice amounts at individual lines.

As a result, unless a sharp eyed human enters an invoice, there will always be a difference between sum of line level amount and invoice header amount.

How can we disable or gray out or just automatically calculate the invoice amount at header level?