Amount  

This is an optional field. It defaults to Quantity * Rate rounded to the precision of the system currency. You can directly enter/override the amount. If you override the calculated value, the system will not recalculate the rate. The Document Totals section at the top of the page will get updated with this amount. The amount can be positive, negative,  or zero.

The sum of all detail line amounts cannot exceed the Request Amount (from PR summary).

Changes to quantity or rate should trigger recalculation of amount value if both quantity and rate are specified. However, changes in amount value do not trigger any other changes.